One day last week, as he watched Sunny's Halo graze on a patch of grass at Pimlico, a worried look came over David Cross' face. "His attention span is nothing," the trainer fretted. "He's too lackadaisical. I don't like the way he's acting."

A hundred other horsemen might have looked at the animal and noticed nothing amiss, but when a man is training a Kentucky Derby winner for the Preakness, he has to be attuned to every nuance. "With horses like this," Marfa's trainer, Wayne Lukas, said, "you know them as well as a husband knows his wife."

There is no type of pressure in sports quite like the pressure that horse trainers must endure before a major race. It's tough enough for human athletes to get themselves ready for an optimal performance when they know first-hand their own physical and mental condition. But a trainer can't ask a horse how he is feeling; he can tell only by watching him closely, day by day, sometimes hour by hour. He cannot afford to let his attention lapse.

Perhaps that is the reason why Cross has never permitted himself to revel in the wake of his Derby triumph. When they were drinking champagne in Barn 41 at Churchill Downs that night, Cross didn't partake; he stuck to his usual Scotch. He has never appeared elated or even relieved that he won the most famous horse race in the world--even though no trainer has ever had more of a right to feel proud of his achievements.

Cross devoted months to the single-minded preparation of this one horse, abandoning the rest of his stable in the process. He formulated a training schedule that was subjected to constant second-guessing, but he never wavered. And then, in two minutes, he became a successful and wealthy man.

But not a serene man. "When you're so wrapped up in one horse," he said at Pimlico this morning, "you're looking for trouble. It's probably like a football coach before the Super Bowl, worrying that his quarterback is going to get hurt."

What Cross has worried about, mostly, is the skin rash that has bothered Sunny's Halo since before the Derby. To read about this much-publicized affliction, you might think that the colt was a grisly specter covered with cankers. In fact, the problem isn't even visible to the naked eye, but you can feel little bumps when you run your hand over the colt's back and shoulders. The rash itself was not a real problem.

"The real worry," Cross said, "is that it might be something in his system, or that it would get into his bloodstream. Then he could be knocked out for two or three months."

To assess the way Sunny's Halo is feeling, to judge whether the skin disease might affect his performance, Cross watches for all the tell-tale signs with which thoroughbreds communicate.

"The most important thing," Cross said, "is the feed tub--how they eat up. If they leave part of the mash, something is the matter. You look at the brightness of their coat--they can change right now. You know the way they act in the stall, how they stand there. And--some people might laugh at this--you look at the brightness in their eyes. You take their temperature; I've been running a temperature on Sunny's Halo four times a day."

Sunny's Halo has been giving Cross all the right signs. For all the publicity about his "ringworm"--which really isn't ringworm, but a type of fungus--he is coming into the Preakness as well as any trainer could want. The skin problem remains strictly cosmetic.

"I'm not proud of what I'll be presenting on Saturday," Cross said. "But we're running a fresh horse, and I couldn't have asked him to train any better."

If Sunny's Halo does win the second leg of the Triple Crown, Cross still probably won't be celebrating at any champagne parties. Judging from the way he has acted in the wake of the Derby, he will probably remain just as intense, just as preoccupied as long as the horse stays in active training. But eventually he will have the chance to savor the great achievement of his lifetime.

"It still hasn't hit me," Cross said. "But I imagine one day I'll be driving down the road and I'll let out a war whoop because I won the Derby!"