All of this country's big-name jockeys ride in New York and California; the ones who achieve success anywhere else are considered minor leaguers. So the racing world was not terribly impressed when Midwesterner Pat Day won 399 races last year, more than any other jockey in the country.

This year Day is running away with the same title, dominating his rivals in Kentucky, Illinois, Louisiana and Arkansas, providing solid evidence that he is among the nation's elite jockeys. He has gained further recognition as the rider of Play Fellow, the country's leading 3-year-old. Still, a measure of skepticism exists. Wasn't this the same Pat Day who once tried to compete in New York and didn't make it in the big leagues?

Day had grown up in Colorado and started riding at little Prescott Downs in Arizona in 1973. He had no background working with horses, but within a year he was the leading rider at Prescott. From there he went to Chicago and enjoyed further success. And in 1976 he set out for New York.

"After three years," he said the other day at suburban Chicago's Arlington Park, disbelieving his own naivete, "I thought I was ready to tackle New York. When I walked into the jocks' room, everybody looked and chuckled. But when it appeared that I might be getting a toehold, I felt the hostility."

Like many other riders, Day felt that New York was an unusually unfriendly environment, a cliquish place dominated by the Spanish-speaking riders. But he recognizes now that his problems there were largely of his own making. "I was not mentally ready for New York," he said. "It's a professional game there, and I didn't attack it as a professional. I was a horrible loser. When I lost I'd take it out on the horses and all the people around me. I just let little things interfere with my concentration."

After that unhappy venture, Day came back to the Midwest. He married a supportive woman; he was reunited with his old agent; he developed contacts with many top trainers; he honed his riding skills. And, he felt, he grew up. "I've matured mentally a tremendous amount," Day said. "I've gained confidence in myself. Winning races means everything to me, but I still know I can't win every race and I don't let things upset me."

All these changes helped Day put his name in the racing record books on Dec. 31. When he finished riding at the Fair Grounds that afternoon, he had the same number of victories as Angel Cordero Jr. Anticipating that possibility, Day's agent had booked a charter flight to take him to a little track in Louisiana, Delta Downs. It was a dreadful night, and the single-engine plane was buffeted by high winds and torrential rain, but Day said, "There was never a question whether it was worth it." When he wound up in the winner's circle aboard a horse named Dana's Woof Woof, he was America's top race-winning rider.

This year he is almost certain to join the select group of jockeys who have accomplished that feat twice. Day already has won more than 300 races and leads his nearest pursuer, Chris McCarron, by about 40 victories.

Although Day gets his pick of the best mounts at most of the tracks where he rides, his Midwestern base has one drawback: He doesn't get to ride many top-class stakes horses, who are mostly based in California or New York. But this summer he finally got that chance--and made the most of it.

Play Fellow had been a disappointment earlier this season, running poorly in both the Kentucky Derby and the Preakness, while veteran jockey Jean Cruguet had been riding him. When the colt's trainer, Harvey Vanier, came to Chicago for the summer, he gave Day the riding assignment. Most racing experts agree that the difference between one good rider and another will be marginal at best, but Play Fellow seemed utterly transformed when Day took over.

Until then the colt had been one of those lumbering stretch-runners who always seems to get into trouble in the early stages of a race. But Day has never encountered a moment's difficulty. The colt won two major stakes at Arlington, then came to Saratoga for the Travers Stakes, where Day rode him flawlessly to beat Slew o' Gold and his old rival, Angel Cordero Jr.

Day left little doubt that the next time he leaves these cozy environs and attempts to break into the upper echelon of his profession, he is going to succeed.