Bay colt Rombauer won the 146th running of the Preakness Stakes, the second installment of horse racing's vaunted Triple Crown.
The unexpected win means the controversial Medina Spirit won't have a shot at the Triple Crown.
"Oh, what a feeling," jockey Flavien Prat said after the race.
The odds for Kentucky-bred Rombauer were 11-1. His trainer is Michael W. McCarthy, and his owners are John and Diane Fradkin of Santa Ana, California.
"I’m not sure this is really happening," John Fradkin said after the race. "This might be a dream."
The bay colt passed Midnight Bourbon, who came in second place, and Medina Spirit, who came in third, for the win.
The Maryland Jockey Club, which runs the Preakness, said on Friday that the Bob Baffert-trained colt passed all drug tests ahead of the event at Pimlico Race Course in Baltimore.
Organizers had struck a deal with Baffert, the Southern California-based trainer who has won a record seven Kentucky Derby runs, to test the colt ahead of Saturday's race. Baffert-trained horses Concert Tour and Beautiful Gift also underwent three rounds of testing, the jockey club said Thursday.
However, the tests by Industrial Laboratories in Colorado and by the University of California at Davis' Maddy Laboratory haven't quieted critics.
"The status quo remains, and it's business as usual in Baltimore," Animal Wellness Action executive director Marty Irby said in a statement Saturday.
"American horse racing continues to be marred by scandal after scandal and the decision to allow Medina Spirit to run in the 146th Preakness Stakes today is no exception," he said.
PETA weighed in on the controversy on Twitter.
"Any trainer who dopes horses is cheating and risks a lawsuit," the animal-wellness group tweeted.
A group of Kentucky Derby bettors on Thursday filed a class-action suit against Baffert in Los Angeles federal court. It alleges "a pattern of racketeering activity" in horse racing.
"Baffert’s multiple and repeated acts of illegally doping and entering horses into thoroughbred races in the State of California and elsewhere constituted a pattern of racketeering activity," the filing said.
Baffert's attorney, Craig Robertson, said in an email that "the lawsuit is completely frivolous and has zero legal merit. We will be promptly moving to have it dismissed."
Baffert issued a statement Saturday saying he would not be in attendance at Pimlico. "I want to keep the focus on this amazing equine athlete [Medina Spirit] and not me," he said.
On Tuesday, Baffert said Medina Spirit was treated for dermatitis with a topical ointment called Otomax, which contains race-banned betamethasone, thus setting off the positive drug test.
"There was never any attempt to game or cheat the system and Medina Spirit earned his Kentucky Derby win," he said Saturday.
Kentucky Derby organizers suspended the trainer and said if the test results hold, Medina Spirit will be disqualified and second-place finisher Mandaloun will be declared the new Derby winner.
The Preakness was aired on NBC. In early action, bettors made Medina Spirit a 5-2 favorite.