On looks alone, Northern Dancer did not stand out as he entered the starting gate for the 90th running of the Kentucky Derby in 1964. He was the small bay colt from Canada nobody had wanted to buy as a youngster. He had sore feet and a volatile temperament. Still, as he filed into Post 7 and the crowd of 100,000 got to its feet, Northern Dancer was the second betting choice. Churchill Downs anticipated a classic.

Once the gates opened, a horse named Mr. Brick charged to the front, and the pace was quick early on. Mr. Brick was followed by Royal Shuck, Wil Rad and The Scoundrel, who slightly bumped the betting favorite, Hill Rise.

Jockey Bill Hartack, with three Derby wins in his career, later recalled that he had a snug hold on Northern Dancer. He dropped him to the rail where he could save ground.

E.P. Taylor, a tycoon who rolled his inherited brewery fortune into dozens of Canadian companies, had bred Northern Dancer at his Windfields Farm outside Toronto. Canada had never delivered a Derby winner, and millions watched their national pride on television. Taylor, or Eddie to his friends, was a man "who often looks as though one of his many companies has just declared bankruptcy," wrote Whitney Tower of Sports Illustrated.

Northern Dancer was a little runt, born late in the season, on May 27, 1961. When Taylor offered up his yearlings at his annual sale, there were no takers at the base price of $25,000. So Taylor kept him. Nevertheless, Northern Dancer's pedigree was faultless, and for a horse his size he had a large girth — spacious room for heart and lungs. As he grew into his stocky frame on the racetrack, he shocked observers with exceptional balance.

"He has a stride that looks two sizes too big for him, but it is perfectly controlled, like something they do at the Bolshoi Ballet," Charles Hatton, a racing writer, once remarked.

Northern Dancer won his first five of seven races in Canada — the minor leagues — but when he came to New York in the fall of 1963, he proved no northern fluke. He finished the year with two easy victories, giving the impression that there was more to come.

But there were fears: one of Northern Dancer's legs was not perfect, and he ran that fall with what is known as a "quarter crack" in his left front hoof. In order to allow for a chance at the Derby the following May, his trainer, Horatio Luro, a tall, dashing, Argentinean born into a family of horsemen, called upon an inventive California blacksmith who had developed an operation for vulcanizing a split hoof.

It appeared to work.

Northern Dancer returned to the races in Florida early in 1964, and he quickly ran off one win after another against the best three-year-old horses on the East Coast. Luro, better known as El Gran Señor, knew how to get a horse to the Derby. He had won with Decidedly in 1962.

But the Derby of 1964, run 50 years ago this weekend, would in some ways turn out to be one of the most important and telling in horseracing history, its real and symbolic impact felt a half-century later throughout a sport roiled by doping scandals.

Not that the fans in the stands had any idea at the time. After all, few at Churchill — for all the tip sheets and bets sold that day — knew the identity of Northern Dancer's veterinarian.

As the field of 12 horses passed the stands that afternoon in May of 1964, Northern Dancer, in the turquoise and gold silks of his Canadian owners, was five lengths behind Mr. Brick. Hartack, his jockey, kept a close eye on Hill Rise to his outside. Luro, Northern Dancer's trainer, had instructed Hartack to conserve Northern Dancer's energy early in the race and save him for a late charge.

And so Hartack, after a fast opening quarter-mile, was content with Northern Dancer's mid-pack position.

In the week before the 1964 Derby, Luro sought the consideration of one man: equine veterinarian Alex Harthill.

Harthill turned 39 that week but was already a legendary figure. He had by then attended to 10 Derby winners, the first in 1948 with Triple Crown victor Citation. His veterinary lineage traced back four generations; his father and grandfather had also cared for Derby winners and contenders. Harthill's influence was such that he had the extraordinary privilege of his own barn on the Churchill backstretch: Barn 24.

John Veitch, the Hall of Fame trainer of Calumet Farm's celebrated Alydar and later a state racing official in Kentucky, knew Harthill well.

"Alex was a real Damon Runyon character," Veitch told me recently. "He was a brilliant vet. But he'd rather make a dollar on the sly than a hundred bucks on the level."

Harthill, at 80, died in 2005, but, in truth, his epitaph had been written decades earlier: brilliant but controversial. For almost six decades, he was the most desired veterinarian in America. He was investigated frequently, but almost always emerged unscathed and defiant, unafraid of giving what to many seemed like self-incriminating interviews.

The legend of Harthill, then, has percolated over the years in the racing world, a place of cynical assumptions and plenty of enduring mystery.

One certainty is that his undisputed talent in caring for horses was matched by his talents inside the office laboratory he kept across the street from Churchill. In the years following World War II, the science of medicine advanced rapidly, as drug manufacturers churned out remedies for human illness. None of these drugs were ever designed specifically for horses, so enterprising veterinarians like Harthill, an avid reader of human medical journals, were at the vanguard of experimenting with them at the racetrack.

Racing authorities, then or now, have never had much luck in their fight against doping. The first laboratory tests — of the saliva of competitive horses — were not introduced until the 1930s, and only then were mostly intended for three things: morphine, heroin, and strychnine. The tests were aimed at determining whether a race had been affected by a horse having been sabotaged by a rival camp or a stable eager to make a large wager.

But the drugs coming from major pharmaceutical companies after the war were being used to improve a horse's performance, and they were remarkably effective. Orders from veterinarians poured in.

Barry Irwin, a prominent owner in today's racing game, said in an interview that he will never forget something Harthill once told him: "Even though a horse is five or seven times larger than humans, the amount of dope needed to have an effect is so small. An amount on the tip of a match would be enough to flick up a horse's nose to get a spectacular result."

The racing authorities were becoming overwhelmed. There was often a lag of years between the synthesis of a human drug for horses and a test for it. These new medications also raised the question of what amounted to doping: testosterone, for instance, became available to trainers in 1947 and allowed them to add spirit to their geldings. It was harmless. It was effective. Was it doping?

Harthill's innovation in this area had already put him in front of law enforcement. There were two incidents in 1954 — one in Kentucky, the other in Illinois — where he was accused of illegally injecting a horse with amphetamines. The following year in New Orleans, he was arrested for public bribery following an alleged doping plot at the Fair Grounds racetrack. The Thoroughbred Racing Protective Bureau, the sport's investigative arm, regularly watched Harthill during this time. But he always won vindication.

Veitch recalled one famous story in which law enforcement was tipped off that Harthill had medications in his car which were legal in Canada but not in the United States. "The story is he claimed the car was stolen and he had never seen those vials," Veitch said. "He was a brilliant veterinarian," Veitch said. Then he paused. "He loved the edge."

"I don't care where the medication was made, whether it was in Europe or Canada or Mexico, Alex was right on it — and often before it was legal to be used in the United States," Veitch continued. "Day or night, if you called Alex, and he liked you, he was there for you."

Entering the backstretch, Hartack guided Northern Dancer to the outside, away from any prospective trouble along the rail. The fractions were swift: 22 2/5 seconds, 46 seconds, and1:10 3/5for three quarters of a mile. But Northern Dancer was still a very relaxed horse.

The Scoundrel began his run after the leaders and Hartack tracked him, leaving Hill Rise, the betting favorite, behind. It was still a half mile from the finish line, and so Hartack was happy to let The Scoundrel go ahead in order to save Northern Dancer for the stretch. He was only concerned about Hill Rise eventually catching him.

"I was in good shape," Hartack said after the race. "I had a horse who had run easily under a tight hold, I was in front of Hill Rise and I knew I had plenty of run left."

The medical term for it is exercise-induced pulmonary hemorrhaging (EIPH), but horsemen for more than 300 years have known it simply as bleeding, a condition as old as the thoroughbred.

For most of that time, however, the cause of it was unknown. Only in the 1960s did it become obvious that this bleeding came from burst capillaries inside a horse's lungs after great pressure during exercise. Blood can make its way into the trachea and, in the most severe cases, pour from the nostrils. For racehorses — nose-breathing animals — this problem can certainly hamper making it to the finish line.

By the end of the 1960s, furosemide, better known by its trade name Lasix, became the drug to deal with it. It was a diuretic that flushed out a horse's system and relieved pressure on the lungs.

Horses with no business winning began doing so, and even the trainers of horses with no bleeding issues began to give Lasix to them. Why not — one well-regarded medical study showed it could leave a horse 16 pounds lighter on race day

As Lasix use took off, Veitch, the former trainer and racing official said, "In most jurisdictions you had two veterinarians that would certify that a horse was a true bleeder through endoscopic examination. Well, that made two veterinarians liars."

In 1974, Maryland became the first state to allow Lasix to be given on the day of a race to horses with documented bleeding problems, and a line was cut through the history of American racing. That year, Fred W. Hooper, the famous owner, told an industry roundtable: "I don't believe that you can control it (medication) once you open up the door."

Other states quickly followed Maryland. Today, claims that a horse has a bleeding problem are no longer required for the use of Lasix, and at least 95 percent of American racehorses have the drug in their system when they leave the starting gate.

Over the years, there has been speculation that Lasix can cause the bones of horses to become more brittle, and it is generally accepted that horses today run less often than in the past because the dehydration and weight loss caused by Lasix is quite taxing.

None of the concerns have tamed the use of Lasix, however, and scores of other drugs have followed Lasix to the racetrack — almost all of them with an ostensible therapeutic use, and almost all of them vulnerable to being abused. And that abuse, over the last decade, has led to the suspensions of prominent trainers, complex sting operations by animal rights advocates, and congressional hearings into the plague of drugs in racing.

For many, then, Lasix is sort of the godfather of the modern doping epidemic. To this day, while the use of many other drugs has been banned or sharply controlled, almost all efforts to curtail the use of Lasix have been stopped cold.

"Whether they need it or not, who the heck knows?" longtime trainer Jonathan Sheppard once told me. "But it's a preventative thing."

Approaching the quarter pole, Hartack decided it was time to go. Decisiveness in such moments can make or break a race: if you use your horse too soon, then he will grow weary in the final strides. But Hartack knew the horses in front of him had gone too fast to keep going; they would begin to labor, their strides faltering, and drop back quickly. Possibly even get in the way, and put him off course.

He had let those in front do the dirty work, and now was the time to ask Northern Dancer, until then relaxed and unruffled, to turn on his renowned acceleration, surge past The Scoundrel and put daylight in front of the field. And make the few horses who had avoided the suicidal pace come after him. Hartack was Mr. Derby. He could see the Churchill stretch without opening his eyes.

"I knew the time had come to use my speed and use it quickly before Hill Rise could start his own run," Hartack said.

Northern Dancer, all balance and velocity, spun out of the final turn in a sensational 11 seconds for an eighth of a mile. He opened up a little more than two lengths between the quarter pole and the eighth pole.

Harthill's work with drugs and horses in the 1960s seems to have straddled the line between legitimate care and the push for advantage. Even those wary of his motives don't believe he ever intentionally harmed horses. And if what he was giving them violated the spirit of the sport, it wasn't illegal. The authorities could not or did not test for the drugs he was introducing to privileged clients.

"He didn't always live by the rules," his friend Gary Priest, who ran a surgical practice with Harthill, admitted to me. "He'd travel all over the world to find the source of a drug we didn't have in the States, and he'd bring it in and use it on horses he thought would benefit from the treatment."

Harthill was beloved and feared, charismatic and hot-tempered. He counted among his friends Frank Sinatra and Ronald Reagan. But he also sucker-punched reporter Billy Reed of the Louisville Courier-Journal in 1968. (The newspaper did not press charges.)

"He was in demand probably as much as any veterinarian of his era, from the standpoint of horses pointing for major races and had some small infirmity," Veitch said. "He knew what his reputation was. He was really proud of the horses that he had — let's use the world 'helped' — in their careers."

In the days before the 1964 Derby, Hill Rise, Northern Dancer's top rival, looked invincible. He won the Derby Trial on theTuesdayof Derby week, and jockey Bill Shoemaker opted to ride Hill Rise over Northern Dancer in the big race, the Derby itself.

Still, Luro, who had saddled winners for all the big prizes, appeared confident in Northern Dancer before the race. Luro, who died in 1991, felt the winning horse would have to travel the final quarter mile in 24 seconds, and that only his horse had the speed and freshness to do it.

During these days, Harthill had his normal clients, but then his special ones too. Luro was a close friend.

"I don't know how to say this," Veitch said, "but he wasn't going to share his secrets with everybody. I don't think Alex was averse to making a wager himself. And if he was sharing those secrets with everybody, it would make it harder for him to have a special horse in that race."

For Northern Dancer, it appears Harthill had Lasix. Veitch said Harthill told him he had been able to get his hands on the drug even before it had been approved for use in humans. The first mention of furosemide appeared in a German medical journal. Trials had evidently been ongoing for several years for German patients with heart problems.

A dozen years ago, Harthill told Jay Hovdey of the The Daily Racing Form that he had given the drug to Northern Dancer before the Derby. Still, how he had obtained a newly synthesized drug is not known.

"Alex was very secretive about his sources," Veitch said. But Veitch, in an interview several weeks ago, said Harthill had openly told him he had used it on Northern Dancer.

Dr. Robert Copelan, a contemporary of Harthill's still practicing after 61 years, told me he first heard about this new injectable diuretic at a veterinary conference in the fall of 1965, which makes Harthill's possession of it in 1964 not out of the question.

It is unclear if Northern Dancer was a true bleeder. His performances on the track — except for a third-place finish against weaker company in early 1964 — suggest he may not have bled badly, and Harthill had other reasons for administering it. Veitch said Harthill had told him that Northern Dancer was hot-blooded and that the diuretic would lower his blood pressure and calm his volatility. He wouldn't leave his race in the paddock, as horsemen say.

"I'm sure they might've experimented with it on Northern Dancer before to make sure it didn't have an adverse effect on him," Veitch said. "They wouldn't go into an event like that without giving it to him and seeing how he reacted, whether it would benefit him or not."

In the interview with TheDaily Racing Form in 2002, three years before his death, Harthill described a certain cloak-and-dagger flavor to his doping of Northern Dancer in the hours before the Derby in 1964.

Track security, he said, had been following him.

"So I got a vet I knew from out of town to come along with me," Harthill said. "I told him I was going to turn to the right, and would he go that way and take this little syringe down to barn 24, stall 23, and give this to that horse. There would be a guy there called Will. He'd be waiting.

"So he did it, while the gendarmes followed me."

On May 2, 1964, just past 4:30 in the afternoon, Northern Dancer hit the top of the homestretch at Churchill Downs.From the final turn to the finish line was 1,235 feet.Hartack, atop Northern Dancer, did not know where Hill Rise was. But Shoemaker and Hill Rise had finally found room, and they came charging. Still, time was short.

Hill Rise had won eight in a row, and surely now he would catch Northern Dancer. The infield crowd swelled to the rail. It was the two-horse race everyone had expected. The rest of the horses were toiling, as Northern Dancer burst from the pack, and Shoemaker uncorked Hill Rise to his outside.

With a sixteenth of a mile left, Northern Dancer's two-length lead was cut to one. Hill Rise moved powerfully and straight. Hartack pushed Northern Dancer, so game he never needed the whip.

At the wire Hill Rise could only reach Northern Dancer's neck. As Luro, the trainer, had plotted, the final quarter was run in 24 seconds; the final time of two minutes was a Derby record.

Hartack had won his fourth Run for the Roses, Luro his second, and E.P. Taylor had bred the first Canadian winner.

Noreen Taylor was 17 when she watched, on television, Northern Dancer win the Derby. She later married into the family that owned the horse, and she learned that her husband, Charles Taylor, the son of E.P. Taylor, had in 1964 been a reporter for Toronto's Globe and Mail newspaper in communist China. E.P. notified his son of Northern Dancer's win via telegraph.

It does not seem from the public record that either the owner or trainer of Hill Rise ever commented on Harthill's claims about using Lasix, and recent attempts to track down anyone who was close to Hill Rise at the time proved fruitless.

Four years ago, Noreen Taylor and the family estate sold Windfields Farm, once home to over 600 racehorses. But some of Noreen's time, she said, is still committed to the remembrance of Northern Dancer, a member of the Canadian Hall of Fame. Books and films are still being made on his life.

As for the role Alex Harthill played in all of this, Taylor said she could not say. In her encounters with Harthill, she said, the subject of doping never came up.

"And yet I know that he has said it," Noreen said of Harthill's boast. "But there is no record we have of any Lasix being administered, or there is nobody I could even call and ask. But he was a terrific veterinarian. It unfortunately just falls within the mists of time."

She did say Northern Dancer could be difficult, so much so that Luro suggested gelding the colt before he ever raced — an idea, it turns out, that E.P. Taylor wisely overruled.

Northern Dancer won the Preakness after the Derby, but lost his Triple Crown bid at Belmont Park. He won the Queen's Plate in front of his beloved Canadian fans, and then an injury ended his career. Like a starburst, it was over in 10 months.

But in a way, it was only a preview. Northern Dancer became one of the most iconic stallions of the 20th century, known especially for his European progeny, and for decades his yearlings sold for eye-popping figures. His sons became champions, and their sons champions. His bloodlines today span the globe.

And much of all that history turned on the outcome of one race, the length of a neck, and maybe a little help.