American Pharoah has trainer Bob Baffert back in the Triple Crown groove.

With thunder rumbling and rain pouring down, Kentucky Derby winner American Pharoah took the lead early and easily won the Preakness Stakes on Saturday (Sunday AEST).

The brilliant colt was never seriously challenged after shaking off long-shot Mr. Z early to win by seven lengths and set up the ultimate drama in racing - a Triple Crown attempt at the Belmont Stakes in three weeks in New York.

"Great horses do great things," Baffert said after his sixth Preakness victory, "and he showed that today."

American Pharoah, who started from the rail under Victor Espinoza, will be 14th Derby-Preakness winner to take a shot at becoming the first to sweep the Derby, Preakness and Belmont since Affirmed in 1978.

Baffert will be taking an unprecedented fourth shot at racing's most coveted prize.

The trainer won the Derby and Preakness with Silver Charm in 1997, Real Quiet in 1998 and War Emblem in 2002. All three fell short in the Belmont.

A rainstorm began about 15 minutes before the race, preceded by several lightning bolts, but it didn't seem to affect American Pharoah.

The colt, who has cotton placed in his ears to keep him calm, dealt with similar conditions to win the Rebel Stakes in March.

American Pharoah thrived in the sloppy conditions, and made it look like a breeze for Espinoza, the first jockey to have a third try at a Triple Crown.

Espinoza won the first two legs with War Emblem in 2002 and California Chrome last year, but fell short in the Belmont.

"I always told everybody American Pharoah would show up today," owner Ahmed Zayat said.

"Indeed he did. He is the real deal."

American Pharoah goes to the 1-1/2-mile (2400m) Belmont on a six-race winning streak. His margin of victory in the 1-3/16th mile (1900m) Preakness was the largest since Smarty Jones won by a record 11-1/2 lengths in 2004.

Tale of Verve was a distant second, followed by Divining Rod, Dortmund, Mr. Z, Danzig Moon, Firing Line and Bodhisattva. The winning time was a slow 1:58.46.

"It went very well," Espinoza said. "He bounced out of there. He broke a little tiny bit slow and I pushed him to the front."

"I didn't know where everyone else was but I didn't want to get dirty."