LOUISVILLE, Ky. - The death of the filly Eight Belles during last year’s Kentucky Derby, along with the revelation that Big Brown had been treated with steroids before his dazzling victory, spurred pledges of reform and accountability for the welfare of the American thoroughbred. But as racing prepares for its biggest show on Saturday, many top owners and trainers still resist even discussing what legal medications their horses are receiving.

Of the 20 owners who as of Monday intended to run a horse in the Derby, only three were willing to reveal veterinary records to The New York Times, showing what legal medications their colts were receiving.

Even with the prohibition of steroids in the past year, the United States continues to have the world’s loosest medication policies for thoroughbreds, and there is a growing concern within the veterinary community that the combination of overmedication — with drugs like corticosteroids, an anti-inflammatory that can have dangerous consequences — and lax oversight have imperiled the safety and welfare of racehorses.

The aggressive use of legal drugs is one of the main reasons this country has the worst mortality rate for thoroughbreds, veterinarians say. In effect, they say, short-term fixes with potent legal drugs have left horses vulnerable over the long term.

The 17 owners unwilling to show the records offered a wide variety of reasons for their refusal. Some talked about competitive pressures, one trainer cited his horse’s privacy, and David Lanzman, co-owner of the likely Derby favorite, I Want Revenge, referred the inquiry to his trainer, Jeff Mullins.

“I’m the wrong guy to talk to,” Lanzman said. “I’m a mortgage banker. I don’t know what goes on back there.” Mullins declined to provide the records.

The owners’ responses make it impossible to tell what practices even racing’s most prominent and accomplished people follow when it comes to using chemistry to improve their horses’ performance.

“We’ve got to quit giving these horses drugs they do not need, and show the public that we are serious about cleaning this sport up,” said Rick Porter, who owned Eight Belles and co-owns the Derby contender Friesan Fire. A necropsy of Eight Belles showed that the filly had no history of a pre-existing injury or condition and was free of steroids and other drugs.

Porter provided the records for a dozen of his horses currently training with Larry Jones. Joining him in offering their horses’ full medical records — which are not required to be disclosed publicly — were Jerry Myers, the owner of Win Willy, and Bo Hirsch, who campaigns Papa Clem. On Wednesday, Win Willy’s owners withdrew him from the Derby when they discovered a possible crack in his left front ankle.

According to the records, each of those owners took a conservative approach to medication when it came to their horses. Porter, for example, paid $165 last month for Friesan Fire’s veterinary care. Some horses’ monthly bills surpass $1,000 for routine care.

The 17 who declined to make their veterinary records available includes names that read like a who’s who of thoroughbred racing:

• Edward Evans is the owner of the Florida Derby winner Quality Road, who had been among the Derby favorites until his scratch on Monday. Evans is one of 98 members of the Jockey Club, one of the sport’s oldest and most prestigious organizations, dedicated, it says, “to the improvement of Thoroughbred breeding and racing.”
• Bill Casner and Kenny Troutt, owners of WinStar Farm, which will enter three horses in the Derby, are trustees of the Thoroughbred Owners and Breeders Association, which is dedicated “to improve the economics, integrity and pleasure of the sport.”
• Sheik Mohammed bin Rashid al-Maktoum, the ruler of Dubai, who has spent hundreds of millions of dollars at American horse sales, will send two runners in search of his first Derby victory on Saturday.

The drug issue has always been contentious in horse racing, and what comes off the back of a veterinarian’s truck and into a horse’s system is often perceived to be as important to the performance of a horse as its talent or exercise routine. Before the 2005 Derby, Churchill Downs and Kentucky racing officials implemented 24-hour surveillance and implemented more comprehensive testing for illegal drugs.

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Last spring at Keeneland, however, the trainer Patrick Biancone, who finished second in the 2004 Derby with Lion Heart, and his veterinarian were barred from the Kentucky circuit when cobra venom — a powerful, illegal painkiller — was found in his barn.

The Hall of Fame trainer Jack Van Berg told a Congressional subcommittee last June that training horses had become “chemical warfare.” Arthur Hancock, a fourth-generation breeder and owner, said that after routinely receiving medication bills for more than $1,000 per horse, he instructed his veterinarian to give his horses only medication when they were sick.

“You want to win races, don’t you, Arthur?” Hancock said the vet replied.

Thoroughbreds routinely receive medications for a variety of ailments, whether the horses have them or not. Mullins, the trainer of I Want Revenge, for example, will start serving a seven-day suspension the day after the Derby for giving one of his horses a cough remedy before a stakes race last month at Aqueduct racetrack. He acknowledged that his horse did not have a cold, but said he gave the treatment to all his horses in the hope of increasing their air capacity.

There are drugs that help prevent ulcers, treat inflammation and open air passageways. One of the most common drugs given to a horse is Lasix, which is administered on the day of a race, a practice prohibited in Europe and Hong Kong. It is designed to prevent blood from seeping into the lungs and airways, and is given to nearly every horse that goes to post in the United States.

Most regulators and veterinarians, however, say it is essentially a performance-enhancing drug because it is a diuretic that flushes 20 to 30 pounds of water out of a horse, making it lighter and faster.