Horse racing has been tarnished by drug scandals for years, but the sport's boosters maintained that these were only isolated occurrences. A few unscrupulous trainers and veterinarians were rotten apples spoiling the whole barrel.

But on Saturday morning at Keystone Race Track in Philadelphia this apologia was exposed as fiction. The day's events demonstrated how widespread and routine the use of illegal drugs has become. It would hardly be hyperbolic to say they suggested that racing is corrupt to the core.

Racing officials in Pennsylvania started hearing rumors weeks ago that a painkiller named Banamine was the latest illegal drug to come into vogue at the nation's thoroughbred tracks. Developed in 1977, it was four times as powerful as the potent drug Butazolidin.

"After we got wind that it was being used," Hart Stotter, racing commission chairman, said, "our people at the testing laboratory worked day and night to find a test for it." The chemists finally succeeded, and came up with "positives" on three horses who had raced at Keystone as well as others at Pocono Downs.

Keystone's stewards received the laboratory report late Friday afternoon and summoned the trainers of the three horses. Steward Louis Baranello said, "As the conversation went on, the trainers indicated matter-of-factly that this had been a treatment wherever they had been racing. Most of our horsemen had come from Delaware Park, Atlantic City and the Meadowlands. We talked to the vets who had treated the horses and they admitted it was a very common procedure.

The stewards surmised that some of the horses who had been entered on Saturday's racing program might already have been given the drug. So they contacted all the veterinarians practicing at Keystone, and told them that if they had administered Banamine they had better advise the trainers to scratch the horses.

The results of this warning were astonishing. Every veterinarian admitted he had administered the forbidden painkiller that day. Of the 90 horses entered on the Keystone program, 20 were scratched because they had been illegally drugged.

This scandal confirms suspicious souls' darkest fears about horse racing. The stringent antidrug regulations adopted in Pennsylvania, Maryland and other states are virtually meaningless as long as pharmaceutical companies keep developing new, potent medications that the racing laboratories cannot detect. With veterinarians pushing the drugs and trainers unwilling to concede any edge to their rivals, the use of substances like Banamine and even more powerful narcotics has probably become standard operating procedure at tracks across the country.

The fact that flagrant violations of the rules were so commonplace gives Pennsylvania officials some special problems. If one or two trainers or vets had been using a forbidden drug, the authorities could feasibly throw the book at them. But if they hand out the warranted suspensions in this case, there aren't going to be many vets or trainers left in business at Keystone.

"This upsets me tremendously," said Stotter, who took over the racing commission chairmanship only last month and has quickly lost his innocence. "The veterinarians obviously knew what they were doing. I've never faced anything quite like this and we're not sure quite what to do."