The 3-year-old colt was scratched Saturday morning after trainer Jeff Mullins discovered a hot spot on the horse's left front ankle.
An X-ray and ultrasound test did not find any damage, but the ankle was tender when it was flexed. With a wet track expected, Mullins and owner David Lanzman didn't want to take any chances.
"When the word came out that running could hurt the horse, I looked at both doctors and said, 'Then this is no debate,'" Lanzman said. "'What are we talking about? We'll fight another day.'"
Larry Bramlage, the on-call veterinarian at Churchill Downs, said the horse didn't look injured while jogging for doctors.
"Unfortunately, this close to the Derby, there's not a way to gauge how bad that is," he said.
With I Want Revenge out, Friesan Fire takes over as the morning-line favorite. He had already become the betting favorite Friday night.
Since the morning line was put in the racing program in 1949, no other favorite had scratched on race day. The last major contender to scratch was second choice A.P. Indy in 1992.
I Want Revenge and 19-year-old Joe Talamo were to start from post No. 13. The duo established themselves as one of the Derby favorites after a last-to-first dash in the Wood Memorial last month.
"I'm just glad the horse is OK," Talamo said. "It could have been a lot worse. Something could have happened on the track. I'm just glad it happened in the stall."
Mullins said he doesn't expect the injury to be career-threatening but will give I Want Revenge some time off before deciding what to do next.
"I've been in this business kind of all my life," he said. "Most of the things I've learned in this business I've learned by hard knocks in more ways than one. Your biggest dream is to get here, but the biggest nightmare is to get to race day and have to scratch. Right now I don't think it's really sunk in that much, but pretty disappointing."
Mullins begins serving a seven-day suspension at 12:01 a.m. Sunday for administering an over-the-counter medication to another of his horses, Gato Go Win, in a detention barn just before a race in New York several weeks ago.
A focus on racehorse safety has been heightened this year following last year's breakdown of filly Eight Belles, the first horse euthanized at the Derby in 134 runnings of the race. The death of that horse sparked changes in the sport, including bans on whips, the padding of starting gates and close monitoring of track conditions. Post-race drug tests on the top four finishers also now screen for steroids for the first time.
I Want Revenge was still at the barn late Saturday morning. There were no immediate plans to take him to an offsite clinic.
"If you walk by his stall, you're not going to know anything is wrong with him," said veterinarian Foster Northrop, who treated the horse. "He's bucking and kicking. He doesn't even know he's hurt."
By Associated Press Writer Jeffrey McMurray;