LOUISVILLE – The 131st running of the Kentucky Derby yesterday was probably the cleanest, squarest, most honest Run for the Roses ever, with all horses competing on a level playing field, free of the influence of illegal drugs.

Never in Derby history has the field of horses been subjected to such intense scrutiny, police supervision and wide-ranging testing for performance-enhancing substances that have plagued racing for half a century and led to a deep distrust by millions of race fans.

But yesterday, as horseplayers shoveled $100 million on the Derby alone and another $50 million on the other 10 races at Churchill Downs, they could be reasonably sure everything was being done to protect the integrity of the contests out on the track.

For the first time, every horse in the Derby was required to submit a blood sample before the race, to be tested for the presence of illegal alkalizing agents, which can be pumped into a horse’s stomach in an attempt to avoid fatigue.

This practice, which is also suspected of masking other more potent drugs, is known as “milkshaking” – and it has been going on for years without scrutiny and known to have affected the outcome of thousands of races all over the country.

After the Derby, more blood samples were tested for no fewer than 140 drugs suspected of being used by unscrupulous horsemen. Last year, Derby horses were subjected to only 30 drug tests.

The milkshake problem, in which baking soda (sodium bicarbonate) is administered to horses a few hours before their races, exploded last summer at Del Mar when the club began testing horses in two races each day without warning. The results were stunning. Some 19 of the 82 samples taken – nearly 25 percent – showed the presence of the illegal stuff.

The problem hit the headlines with a thump in February when one of California’s best known-and biggest winning-trainers, Jeff Mullins, was found to have five horses with bicarbonate levels exceeding the legal limit. Mullins was the leading trainer at Santa Anita and Hollywood Park last year. Another prominent trainer, Vladamir Cerin, was also flagged.

Both trainers were ordered to put their horses in a specially protected detention barn 24 hours before their races for a minimum of 30 days. Mullins, who had been winning races at an almost unbelievable 30-percent clip, suddenly saw his winning percentage sink to 8 percent.

In March, the milkshake scandal exploded. A Los Angeles Times columnist, T.J. Simers, interviewed Mullins. He reported that Mullins told him, “The problem is all the addicts and idiots crying because they lost $2.”

Mullins then added, “If you bet on horses, I would call you an idiot.”

His contempt for horseplayers, who only keep the game going, infuriated the whole industry. At first, Mullins fell back on the old reliable, claiming he had been misquoted. But later he conceded, “I obviously said some things I didn’t mean and should not have said. Believe me, if I could take those things back, I would.”

Soon after, Mullins rocked the racing world again when his horse Buzzards Bay, considered a forlorn outsider, steamed home to win the Santa Anita Derby at better than 30-1. Buzzards Bay was scheduled to run in the Kentucky Derby yesterday.

It was the third straight year Mullins had won the Santa Anita Derby with longshots. His other two winners, Buddy Gil and Castledale, were both brought to Louisville for the Kentucky Derby, only to finish up the track.

When he pulled into the Downs’ backstretch late last week with Buzzards Bay, Mullins kept his lips sealed and refused to discuss his problems. “I’m not doing many interviews,” he said, “and if you guys don’t know why, I don’t need to tell you.”

Still, that may not erase his new nickname among the cognoscenti, who now call him “Milkshake Mullins.”

Last week, Belmont Park introduced tighter security measures against horse drugging. All horses must he taken to a security barn at least six hours before their races. Trainers and their employees may remain with the horses during detention, but private veterinarians are barred.

Trainer Phil Serpe told the Daily Racing Form, “People are breaking the rules and everybody knows it. To me, it’s a near out-of-control situation. People are being called great trainers and heroes of the game and they are not.”

Most horsemen favor the drug crackdown. Said top trainer D. Wayne Lukas, “We want to get the credibility back. People are betting on our horses and we have to give them something to feel confident.”

The new regimen has begun and there is no better place to start than at the biggest race of them all, the Kentucky Derby.