Orb powered through the mud to win the 139th Kentucky Derby and give Hall of Fame trainer Shug McGaughey his first winner in the Run for the Roses.

Jockey of the moment Joel Rosario piloted Orb to the victory in the 1-1/4-mile (2000m) race on a muddy track at Churchill Downs on Saturday (Sunday AEST) ahead of long shot Golden Soul and Revolutionary.

Rosario, who rode Animal Kingdom to victory in the $US10 million ($A9.8 million) Dubai World Cup in March, followed up that international triumph with a superbly patient victory in America's most celebrated race.

He let Orb settle back in the field as Palace Malice set a blazing early pace, saving ground around the first turn to have plenty in reserve for a dominant stretch run.

"This guy today, it was all him," Rosario said, patting his mud-splattered mount.

"I was so far behind and I just let him be calm and let him be relaxed and he was available to do it all."

Sent off the 5-1 favourite, Orb surged past Normandy Invasion in the final straight and held off 34-1 shot Golden Soul to win by 2-1/2 lengths.

"It's awesome," Rosario said. "I won the Dubai World Cup, and to win the Derby now it's like a dream. I'm so happy for Shug McGaughey."

McGaughey, a Kentuckian, had never managed to get to the winner's circle at the Kentucky Derby in six prior attempts.

"It means everything to me," he said. "I always dreamed of this day."

Revolutionary (6-1) ridden by popular Calvin Borel and one of five horses in the field of 19 trained by Todd Pletcher, was third a length behind Golden Soul and just a head in front of the tiring

Normandy Invasion.

It was a fifth straight victory for Orb, winner of the Florida Derby and Fountain of Youth Stakes at his two previous starts.

With the win, Orb gains a chance to become the first horse since Affirmed in 1978 to capture US flat racing's Triple Crown of the Kentucky Derby, Preakness and Belmont Stakes.

Only 11 horses have completed the coveted treble.

Steady rain pelted Churchill Downs for much of the day, making for a sloppy dirt track even though it had tapered off by post time.

The weather didn't dampen the enthusiasm of 151,000 spectators, nor did the extra security measures put in force in the wake of the deadly bombing at the Boston Marathon last month.

Churchill Downs urged spectators to be aware of "unusual or suspicious" behaviour amid the party atmosphere that rules the infield, and also revised their list of banned items to include refreshment coolers, large bags and metal drink cans.