Spend A Buck was greeted by some skepticism when he came to Churchill Downs, because it seemed impossible that he really could be as fast and as brilliant as he had looked in his most recent race.

Now the world knows: He is. The colt ran the third-fastest race in Kentucky Derby history today and demolished what was supposed to be one of the most competitive fields in years.

Leading all the way under jockey Angel Cordero Jr., he scored a 5 1/4-length win over Stephan's Odyssey. It was the biggest margin of victory in the Derby since 1946. Chief's Crown was third.

Last month, Spend A Buck narrowly missed Secretariat's world record for 1 1/8 miles when he won the Garden State Stakes in similarly stunning fashion. But he didn't have competition for the early lead that night. In the Derby he was meeting an equally fast rival in Eternal Prince, and there was a widespread expectation that they might engage in a destructive fight for the early lead.

But it never materialized. What happened in the first sixteenth of a mile determined the nature of this entire Derby.

When the gate opened, Eternal Prince's jockey, Richard Migliore, said, "He just didn't break. He was kind of upset by the crowd all yelling and screaming."

Migliore did not push him aggressively in an effort to get to the front, and quickly found himself engulfed by other horses. "By the time we were at the first turn," Migliore said, "he was trying to run up on horses' heels. He had no clear running space."

Cordero, at 42 the oldest jockey to win the Derby, couldn't believe what was happening. "I didn't expect Eternal Prince to be that far back," he said. "I didn't want to fight with anybody."

When he saw no competition for the lead, he angled Spend A Buck to the rail and was clearly in front after a slow, 23-second quarter-mile. The 111th Kentucky Derby essentially was over.

Don MacBeth, on 6-to-5 favorite Chief's Crown, had envisioned sitting comfortably behind a duel of the two speed horses, but now he knew he had to chase the front-runner.

But Chief's Crown doesn't have that kind of raw speed; not many horses do. Spend A Buck ran away from him, reaching the half-mile mark in :45 4/5 as he opened a six-length advantage.

All MacBeth could do was sit in second place and hope the leader would collapse. He never had a prayer.

Spend A Buck continued to accelerate, hitting the three-quarter mark in a blazing 1:09 3/5 and the mile in 1:34 4/5, the fastest fractions in Derby history. He covered the 1 1/4 miles in 2:00 1/5, faster than every Derby winner but Secretariat (1:59 2/5 in 1973) and Northern Dancer (2:00 in 1964).

"When we went to the stretch he was jumping at shadows and getting nervous. I had to spike him to keep his concentration," Cordero said. "I was not surprised that this horse ran so well, but I was surprised he won so easily."

Chief's Crown weakened near the end, and Stephan's Odyssey rallied up the inside to take second by half a length. Fast Account was another neck back in fourth place.

Of the other horses considered contenders, Proud Truth finished fifth, Tank's Prospect seventh, Rhoman Rule ninth and Eternal Prince 12th in the 13-horse field.

Spend A Buck paid $10.20, $5.40 and $3.40. Stephan's Odyssey returned $10.20 and $5. Chief's Crown paid $2.80 to show. The exacta was worth $118.20.

Cordero now has won three Derbies. He left the track almost immediately to fly to New Jersey. He was scheduled to ride Kattegat's Pride in the $200,000 Betsy Ross Handicap at Garden State Park, but he arrived too late.

Probably no horse came into this Derby with such modest origins as Spend A Buck. Dennis Diaz, who recently retired from the insurance business at 38, bought the son of stallion Buckaroo for $12,500 and turned him over to a young, little-known Florida trainer, Cam Gambolati.

Spend A Buck started his career in the humble environs of Miami's Calder Race Course, but he kept winning and winning, earning more than $600,000 as a 2-year-old and finishing third behind Chief's Crown in the Breeders' Cup.

Still, even Gambolati thought the speedster was basically a miler. Even after Spend A Buck won the Cherry Hill Mile at Garden State, neither the trainer nor the owner was thinking seriously about the Kentucky Derby. But after his 9 1/2-length romp in the Garden State Stakes, Gambolati knew he had no choice but to come to Louisville.

Now he does have a choice. Because of his triumphs in the other Garden State races and the Derby, Spend A Buck is eligible for a $2 million bonus if he wins the Jersey Derby at Robert Brennan's new track on May 27. That is nine days after the Preakness, and Gambolati said immediately after today's victory, "There's no way we can run in them both." He and Diaz will decide which race to run in the next day or two.

"I don't know what they have planned for him," said Cordero, "but wherever this colt goes I want to be there."

Should they take an easy $2 million at Garden State, or pursue a Triple Crown sweep that might make their colt worth more than $20 million as a stud prospect? It's the kind of dilemma that everyone who has ever bought a cheap horse would love to have.