Laz Barrera has dominated the training profession so thoroughly in recent years that it almost seems he has dominated it forever.

He has won the Eclipse Award as the outstanding member of his profession four times in the last seven years. His horses have earned nearly $20 million and six Eclipse Awards during that period, and Lemhi Gold could add to the list if he is voted the Horse of the Year for 1982.

Yet in 1976 Barrera was a relatively minor trainer, a man who had turned 50 without making any indelible impressions on the sport. In this game, though, a man's fortune and reputation can sometimes be made or broken in a single race, and Laz Barrera's entire life was altered at Churchill Downs on the first Saturday in May 1976.

Barrera had come to the Kentucky Derby with his colt Bold Forbes, who was the prototype of the horse who cannot win at the Derby distance. Horses like him, with brilliant, uncontrollable speed and a doubtful pedigree, collapse in the stretch at Churchill Downs every year. So his trainer attempted to change Bold Forbes' very nature.

He put the colt through a regimen of long, slow gallops and workouts that were designed to build his stamina and teach him to relax. Barrera professed confidence that his experiment would succeed, but nothing in his or his horse's record suggested that it would.

When Bold Forbes led all the way to upset Honest Pleasure, and then confounded the pedigree experts by winning the 1 1/2-mile Belmont Stakes, Barrera was hailed as a genius. And Barrera felt a strong sense of self-esteem, too. "I knew I'd done something," he said. He had the self-confidence that would be the foundation of his incredible success.

Certainly, he had waited long enough for it. A native of Havana, Barrera trained horses in Mexico City for nearly 15 years. "Finally I said, 'God Almighty! I can't wait any longer!' and I came to this country. But it took me another eight or 10 years to let people know who I was. I was training mostly for people from Latin American countries. But I wasn't frustrated. I knew one day I'd get the horse I wanted."

After getting the horse he wanted, Bold Forbes, Barrera got the horse who was beyond his wildest dreams: Affirmed.

In the wake of his great accomplishments--the Triple Crown, championships at ages 2, 3, and 4--racing people forget that Affirmed's stamina was initially suspect, too. He had a sprinter's speed and a pedigree of doubtful stamina. But Barrera put him through the same course of study that Bold Forbes had passed--those long, slow workouts--and Affirmed was a model student. Barrera managed him flawlessly through his epic confrontations with Alydar to his final great triumph over Spectacular Bid, but the job exacted a heavy toll on him.

Anyone who watches Barrera at the race track for more than a few minutes can feel his intensity. He doesn't train his horses according to a manual. He decides what to do by watching them, watching them gallop on the track, watching them walk around the barn, examining every motion and every centimeter of their bodies, looking for clues to what the animal is feeling and what he needs. The demands of the job, and the sheer pressure of caring for a multimillion-dollar animal, took their toll on Barrera on the day that Affirmed won the Santa Anita Handicap.

That morning Barrera felt a stabbing pain in his chest, and after the race, while the colt's owners were drinking champagne, the trainer went home and called his doctor. The next morning he was on an operating table undergoing triple-bypass heart surgery. "My triple crown trophy," he calls it.

But if that scare forced Barrera to evaluate his life and career, it didn't motivate him to change it. "I still work seven days a week, and even if I'm not running a horse I come to the track every day," he said. "Racing's my life; there's nothing better in the world. If I die, I want to die on the track."

After waiting so long to reach the summit of his profession, Barrera is not about to abandon it now.