Old jockeys usually fade away, slowly and ingloriously. Although they don't often get a sudden, jolting realization that they have lost their skills (as, say, Muhammad Ali did), time can defeat them in a number of subtle, insidious ways.

Some older riders find they no longer can control their weight. Some lose their reflexes; some verify the old cliche that "the legs go first." And some, having suffered too many harrowing spills, lose their nerve and their willingness to take risks.

Then there is the category of jockeys that is occupied almost exclusively by 48-year-old Bill Passmore, who seems ageless and impervious to the ravages of time.

Relatively few Maryland race-goers can remember a time when Passmore was not winning races regularly at the state's tracks, nursing horses along on the lead, driving them to the wire with that unmistakably fluid style of his. For the record, he has not been riding forever, but only since 1948, when he was 15 and earned his maiden victory at old Jamaica Race Track. Since l958, he has been a fixture in Maryland, and he has ridden more than 3,000 winners in his distinguished career.

Racing fans might have thought that Passmore had reached the end of that career three years ago, when he was involved in a frightening five-horse spill at Pimlico and broke his back. Yet he returned to the track unaffected, won his first race and resumed his old pattern of success. People who were at Bowie on Sept. 21 might also have wondered if they were witnessing a disaster from which the jockey would not recover.

Passmore was astride a filly named Inclavitating as she was being loaded into the gate. But the inexperienced 2-year-old suddenly became frightened, stopped in her tracks, reared in the air and flipped over. Passmore was under her when she crashed to the ground. "I was conscious," he recalled. "I tried to get up, but I couldn't. I knew something was broken."

What was broken was his pelvis, in four places. His sacroiliac was also damaged, and Passmore has been on an enforced vacation ever since. His reaction to his absence from the track probably provides a clue to the reason for his perennial success. He was miserable; after more than three decades of riding he had not lost his enthusiasm for the game.

"I was going nuts in the house," he said. "You know, you hear so many people say they hate their job. I'm not like that. When it starts to become a lot of work for me, I'll quit. But I can't wait to get back."

Passmore shed his crutches and started galloping a few horses for trainer King Leatherbury, who has employed his services since 1975 and holds him in the highest regard. "He's a classic kind of rider, a sit-still kind of rider, very composed and cool," Leatherbury said. "He's not the beat-'em-up type. He holds a horse together real good. He's an excellent hand rider. He has an excellent sense of pace. And on the front end, there's no rider better."

Passmore is scheduled to return to competition today, and he will be watched critically. The track is not a terribly compassionate place; when people are betting their money they do not easily forgive a jockey for his errors, even if the jockey happens to be a local institution.

But Passmore is unworried about how he will perform. "I haven't put on an ounce in the last three months," he said. "I have no problems physically. I think I'm riding as well as ever." He has banished from his mind the thought of the risks involved in his profession and the memory of his previous accidents. "You can't let it bother you. If you let it enter your mind, there's no sense in going on."

Leatherbury has no doubts about how his jockey will perform, either. "It'll be the same old Passmore," he said. Somehow it seems almost inconceivable that Passmore will ever cease to be the same old Passmore.