When the starting gate opens for the 129th Kentucky Derby next Saturday, every horse straining to win will have received an injection of a diuretic before the race. The drug, commonly known as Lasix, controls bleeding in the lungs but also is clinically proven to enhance performance.

Many of the horses will also have received a shot of the anti-inflammatory phenylbutazone, along with an additional powerful painkiller. Until last fall, racehorses in Kentucky could use two anti-inflammatories or painkillers from a list of 16 on race day. Intense negative publicity led the state to reduce the number to two of five.

But even with all the legal medications available to trainers and veterinarians, the sport faces an even greater concern. As heavy drug treatment of racehorses in the United States has become commonplace, illegal drugs continue to be used on the animals, often undetected, according to industry officials, trainers and veterinarians.

Like sports involving human athletes, the use of illicit performance-enhancing drugs threatens the credibility of competition at a time when the industry is still recovering from last fall's Breeders' Cup Pick Six computer wagering scandal. A lack of national oversight and inadequate testing procedures have cast a cloud over a sport in which last year saw bettors wager a record $123 million nationally on Derby day.

"I believe American thoroughbred and harness racing are great sports, and I think a very small number of people are doing this," said Stan Bergstein, executive vice president of the Harness Tracks of America and a leading critic of drug use in racing. "But a small amount of it is too damn much."

Compared to European racing, where all drugs are prohibited, finding an American racehorse trained on the traditional hay, oats and water probably would be impossible. Instead, U.S. racing is governed by a patchwork of guidelines that vary in the 24 racing jurisdictions. In the past year has the industry taken the step of establishing the Racing Medication and Testing Consortium, which hopes to create uniform laboratory technical standards and procedures for testing.

Existing testing procedures, which are capable of detecting if trainers are using too much of an approved drug, are considered inadequate to keep up with horsemen using new substances.

"The evolution of the use of prohibited substances has gotten ahead of the scientists," Bergstein said. "Certain horsemen are willing to experiment with drugs without knowing the effect on the horse."

Said Maryland-based trainer Lori Testerman: "It's no longer my horse is better than yours; it's my vet is better than your's. I don't like to see the shadow on racing. Every time a long shot comes in, they say, 'He's got the stuff.' "

Trainer Mark Shuman and owner Michael Gill have been denied stall space at five East Coast tracks after aggressively claiming horses that later improved their performances markedly under Shuman's care. The duo smashed the record for most wins at the Gulfstream Park winter meet in South Florida. None of their horses tested positive for a banned substance.

The track investigated Shuman and Gill after one of their veterinarians allegedly cut the leg off a 9-year-old gelding named Casual Conflict that had broken down and been euthanized following a race Feb. 3. The two were later cleared of any wrongdoing although Gill's vet, Philip Aleong, was barred by the track, as was another in Shuman's employ, Leonard Patrick, who had violated a drug storage rule.

Gill, by far the leading owner in the country, has a history of horse drug violations. Shuman, before moving from his Maryland base to Florida, had been fined and suspended in September, along with another Gill trainer, for positive tests for a banned substance.

The duo declined to comment but in earlier interviews they have repeatedly denied any wrongdoing.

Even the most sophisticated state test labs only can spot the presence of drugs they already know are in use. Outlaw substances, both known and unknown, are only now beginning to appear on the radar screens of racing labs. The vexing questions are: How many potential performance-enhancing substances are out there, and how much would it cost a lab to test for all of them?

"It could be thousands," Scott Waterman, executive director of the Racing Medication and Testing Consortium, said of the number of potential drugs in play.

Yet, for every accusation that a trainer is cheating, there is a lab test vouching he is not. That doesn't mean the horse is clean, said Thomas Lomangino, director of the Maryland Racing Commission Laboratory in College Park, considered one of the most vigilant labs in the country.

"Just as you wouldn't know me on the street until we've been introduced, I wouldn't recognize a drug until I've seen it," Lomangino said.

Whispers about the use of foreign substances such as cobra venom or cone snail venom, which Lomangino says leave no trace elements two days after being administered, tarnish the reputation of racing when they go unsubstantiated, many in the sport say.

"If you want to tell me someone is using rocket fuel, it doesn't do me any good," said Waterman, a licensed vet. "The trick for us as we try to separate fact from fiction is to try to get good, solid information."

The Racing Medication and Testing Consortium is trying to "winnow out the red herrings and focus on credible threats," Waterman said.

One of the most credible rising threats is Erythropoietin -- or EPO -- a naturally occurring glycoprotein that develops in the kidneys and aids in the production of red blood cells. Artificially produced EPO is used in human patients suffering from anemia, but it has found its way into racehorses even though it is outlawed by every racing jurisdiction in the United States.

EPO is listed as a Class 2 drug by the Association of Racing Commissioners, meaning it does not have accepted therapeutic value in horses, but has a high potential to affect performance.

Lomangino said it is dangerous in high doses and if a horse is taken off of EPO, it could cause a sludging of the blood. "It starts killing horses," he said. "How bad do you want a dead horse?"

There is now a test, not for EPO, but the antibody a horse produces in reaction to its presence, Waterman said.

"They ran this test over two days at Sam Houston [in Houston] and had six hits out of 55 tested," he said. "That's a higher percentage than I would have expected."

Trainers, by rule, have the ultimate responsibility for the condition of a racehorse, no matter who might administer a medication. However, unlike most drugs that leave a trace arc over a period of time, making it easy to identify approximately when they first entered a horse's system, antibodies reacting to EPO may remain visible for months in a horse, Waterman said.

"So, it's almost impossible in a claiming horse that has changed hands a few times to implicate anybody who might have administered the drug," he said. "You don't want to create a system that penalizes innocent people."

With only the antibodies to go by and no idea who administered the EPO, state regulators have chosen not to penalize horsemen -- or release their names -- and instead use the opportunity to educate them on the dangers of the drug.

"We notify the owner that their horse has tested positive, indicating that the horse may have been treated with EPO," said Paula Flowerday, executive director of the Texas Racing Commission. "At this point, we have no way of knowing if that drug was in the horse's system at the time of the race."

This opens the door for easy exploitation.

"When a product comes into use, and it's found there's not a test for it, the product just explodes," said trainer John Ward, whose colt Monarchos won the 2001 Kentucky Derby. Ward is one of two trainers advising the Racing Medication and Testing Consortium. "The grapevine gets going, and a lot of people push the envelope."

Drug use to boost a horse's performance is hardly a recent trend. The only horse ever disqualified in the 128-year history of the Kentucky Derby was Dancer's Image, a Maryland-bred son of Native Dancer, whose post-race urine test in 1968 turned up bute.

The amount found in Dancer's Image after the Derby would be permissible under Kentucky standards today, but at the time was strictly forbidden, costing the horse's owner the first prize of more than $122,000 and Derby immortality.

The Kentucky veterinarian who recommended Dancer's Image be administered the drug the week before the race, Alex Harthill, long has been one of the most sought-after vets at the upper echelon of the racing game but he also had a history of violations. He had been suspended for 60 days in 1954 for a positive drug test, and charged of bribery and tampering with horses the following year at the Fair Grounds in New Orleans (he was not convicted).

Harthill, still prominent today and the former president of the Kentucky Horsemen's Benevolent and Protective Association, has been intensely investigated over the years and the subject of a litany of unsubstantiated stories about his ability to dramatically improve -- and degrade -- the form of horses through medication.

As president of the Kentucky HBPA, Harthill fought hard against uniform medication policies. Harthill resigned in 2002 in disagreement with the Kentucky HBPA board over its decision to halt a bribery investigation. His replacement, Susan Bunning, appears to be behind the push for uniform standards.

Reached in Kentucky, Harthill, now 78 but still active and influential after more than 50 years, said those who cast themselves as reformers are largely ignorant of the needs of the animals.

"I think they're fools," Harthill said. "I think these here people are so ill-informed that they don't have any business adjudicating policy. I want the horse to be able to be treated properly for what's wrong with them."

Reform may take place, with or without Harthill, if the major organizations behind the RMCT can standardize the rules governing medication in each state and then get the backing of racing commissions and horsemen.

But Ward is dubious.

"I think it's easier to bring a democratic government to Iraq than get uniform drug testing," Ward said.

Among its goals, the RMTC aims to reduce the opportunities for racetrack trainers or vets to purchase illegal or banned substances through greater security.

So far, the consortium has begun developing guidelines for uniform standards for drug thresholds, prohibited medications and disciplinary procedures to be adopted by state racing commissions across the country. This is a critical issue because trainers often find themselves penalized for violations when racing in other states, even though they haven't altered their usual medication practices.

Waterman, who puts this year's budget for the RMTC at $800,000, knows the task is great and will take time.

"This is the first year of the organization," he said. "For the time being, I would prefer a lower rather than higher profile. I want the regulatory community to feel they aren't under a threat."

On a day when the name of the Derby winner will be inscribed next to those of past legends such as Citation, Secretariat and Seattle Slew, the sport's crown jewel will be contested in a state with a longstanding reputation for its permissive drug policy.

"I wouldn't call it out of control," said trainer Ward. "It's in more control than it's ever been, but it still has a lenient medication policy."

Bernard Hettel, executive director of the Kentucky Racing Commission, dismissed the criticism. "Generally, people make these claims around Derby time. There is a necessity, from this commission's perspective, to allow horses to use some medication," he said. "We have limited the number of drugs that can be used from 16. . . . Don't go saying that any horse has ever run in our races with 16 drugs inside them. That simply means that there were 16 products that we allowed."

Staff writer Greg Sandoval contributed to this report.

Unsuccessful drug testing procedures, a lack of national oversight have cast a cloud over horse racing and have shaken bettors' confidence. Respected vet Alex Harthill says reformers are largely ignorant of the needs of the animals. "I think they're fools."Maryland-based trainer Lori Testerman: "It's no longer my horse is better than yours; it's my vet is better than your's. I don't like to see the shadow on racing."