In prep races over the next four weeks, most of the country's good 3-year-olds will reveal whether they have the talent and stamina to win the Kentucky Derby. But the nominal derby favorite, Copelan, has already answered all the crucial questions about his ability.

He has answered them negatively, and he will not be draped with roses on the first Saturday in May. Although he will run in important prep races, including the Flamingo Stakes at Hialeah this weekend, and although he will be ballyhooed until the day of the Derby, his ultimate failure is inevitable, inescapable.

Even a casual handicapper could glance at Copelan's record and quickly conclude that the colt will never be able to go the Derby distance. He has tried to run farther than a mile four times and has won only once, under absolutely optimal conditions. Twice, he faded so badly that he finished out of the money.

Yet owner Fred Hooper and trainer Mitchell Griffin are unable to accept reality and keep trying to excuse his defeats. Almost everybody in the racing game makes alibis, of course. But Hooper and Griffith seem to believe their own rationalizations and are planning the horse's future campaign on the basis of them.

Copelan showed that he was a gritty competitor when he won the first six races of his career. But the first time he tried to go 1 1/16 miles, in a stake at the Meadowlands, he tired and finished fourth. Unfazed, the owner and trainer sent him west for the rich Hollywood Futurity at the same distance. Here, he was a victim of the Great Clod Incident.

When Copelan finished fifth, 16 lengths behind the victorious Roving Boy, some observers were uncharitable enough to think that he was a bum. But, no! The day after the race, his veterinarian revealed that Copelan had been hit in the eye by a dirt clod, accounting for the dismal performance. Hooper and Griffin talked about the incident as if a boulder had flattened the colt.

Cynics pointed out that if Copelan had been fast enough to get the lead in the futurity, nobody would have been kicking dirt in his face. But most racing people accepted the clod story and expected the colt to vindicate himself as a 3-year-old.

He did win his first start of the season at 1 1-16 miles when he was able to get an uncontested five-length lead against a field of nonentities. But in the Florida Derby, Copelan clearly revealed his inadequacies.

Copelan sat a length or so behind an 85-to-1 shot, Croeso, as he set a slow pace, challenged on the turn and drew even with him an eighth of a mile from the wire. In an undistinguished eight-race career, Croeso had never passed a single horse in the stretch, but he was able to draw away from Copelan and score a 1 1/4-length victory.

Some observers took this as solid evidence that Copelan can't go a distance. But, no! In the view of Griffin and, indeed, in the view of many reporters covering the race, Copelan lost because jockey Laffit Pincay Jr. blew it.

They maintained that Croeso had "stolen" the race by taking the lead and setting slow fractions, an interpretation that might have been valid if Croeso had been permitted to open a five-length lead. But it doesn't explain why Croeso was able to run away from Copelan after they were head-and-head in the stretch.

Still, Griffin clung to this excuse. "Letting Copelan run the first half in 48 1/5 seconds was the big difference," he said. "It's all water over the dam now, but never again will Copelan let another horse have such an easy lead. Next time, he'll force or make the pace."

Brilliant. One of Copelan's great virtues as a racehorse is his tractability; his jockey can sit behind the leaders and use his speed when he wants.

But now Griffin evidently wants him to rush up and get involved in duels for the lead every time an 85-to-1 shot gets in front of him briefly. He's not going to win the Flamingo or the Kentucky Derby running that way. Griffin and Hooper had better start getting a new batch of excuses ready.