LOUISVILLE, Ky. — Big Brown was pulling away from the field, accelerating with every powerful stride toward the finish line in the Kentucky Derby.

The crowd of 157,770 was on its feet and cheering as the big, unbeaten, muscular bay crossed the line first, 4 3/4 lengths ahead of the filly Eight Belles.

Trainer Rick Dutrow Jr. was still celebrating, along with thousands of happy bettors, as Big Brown and the 19 other horses in yesterday's race galloped out around the first turn at Churchill Downs.

It took a few minutes to sink in, but anyone watching those horses soon realized that one of them had fallen to the track.

"It's the filly," someone whispered. She went down about a quarter mile past the finish line.

In just a few minutes, the joy of the Derby and the promise of a new Triple Crown season were upended when Eight Belles was euthanized by injection on the track.

She had broken both front ankles and could not be saved.

"This horse showed you his heart," winning jockey Kent Desormeaux said, "and Eight Belles showed you her life for our enjoyment today. I'm deeply sympathetic to that team for their loss."

Big Brown did everything his owner said he would do. An explosive finishing kick put away his rivals for his fourth consecutive victory.

Eight Belles, meantime, was attempting to become the fourth filly to win the Derby. Her owners chose to keep her out of Friday's Kentucky Oaks so she could run with the boys in the Derby.

Big Brown's start from the outside post did little to hamper his charge when the field turned for home. Under the urging of Desormeaux, the 2-1 favorite cruised to an easy victory to become the seventh undefeated Derby winner. The last one was Barbaro in 2006.

That wasn't the only reason thoughts of Barbaro were hard to ignore on this Derby Day.

The breakdown brought back memories of the 2006 Preakness, where Barbaro shattered his right rear leg just after the start. The colt was euthanized months later, after developing laminitis from the catastrophic injuries.

In two weeks, Big Brown will race in the Preakness as the only 3-year-old with a chance to become the first Triple Crown champion since Affirmed in 1978.

"We're ready to roll," Dutrow said.

All week, Dutrow told the world he had the best horse in the field — and the big bay colt justified every accolade tossed his way.

"I can't describe the feeling that all of us have right now," he said.

The colt became the first Derby winner since Regret in 1915 to have raced only three times previously. He is only the third in 60 years to win after racing in just two Derby preps — Sunny's Halo in 1983 and Street Sense last year were the others.

In addition, Big Brown became the second winner to start from the No. 20 post. The gelding Clyde Van Dusen did it in 1929.

Big Brown covered the 1 1/4 miles in 2:01.82 in front of the second-largest crowd in Derby history. He paid $6.80, $5 and $4.80.

Eight Belles paid $10.60 and $6.40, and Denis of Cork, at odds of 27-1, returned $11.60.

Dr. Larry Bramlage, the Derby's on-call veterinarian, said the filly's injuries were too severe to even attempt to move her off the track.

"She didn't have a front leg to stand on to be splinted and hauled off in the ambulance, so she was euthanized," Bramlage said.

Trainer Larry Jones paid tribute to his fallen filly saying, "She ran the race of her life."

And he defended having her run against 19 colts in the Derby.

"It wasn't that. It wasn't the distance. It wasn't a big bumping match for her. She never got touched," he said. "She passed all those questions ... with flying colors. The race was over, all we had to do was pull up, come back and be happy. It just didn't happen."

Tale of Ekati was fourth, followed by Recapturetheglory, Colonel John, Anak Nakal, Pyro, Cowboy Cal, Z Fortune, Smooth Air, Visionaire, Court Vision, Z Humor, Cool Coal Man, Bob Black Jack, Gayego, Big Truck, Adriano and Monba.

The colt earned $1,451,800 for the win and boosted his earnings to $2,114,500 for owners IEAH Stables and Paul Pompa Jr. Pompa, who named Big Brown in honor of United Parcel Service, a client of his trucking business, sold a 75 percent interest in the colt to IEAH for about $3 million after his first race.

Desormeaux won the Derby for the third time, having won aboard Real Quiet in 1998 and Fusaichi Pegasus in 2000. Only three other riders have won more — Eddie Arcaro, Bill Hartack and Bill Shoemaker.

"It was smooth sailing all the way," Desormeaux said. "The horse was very comfortable."

Big Brown was the third favorite to win in the past five years. Smarty Jones won in 2004 and Street Sense won last year.

"I don't even know what we just did," Dutrow said. "I can't express my feelings, only that it was one of the most incredible feelings I ever had, and I can't wait to feel it again."