Bob Baffert's National Treasure has won the Preakness Stakes, ending Mage's Triple Crown bid in the trainer's return from a suspension - and just hours after another of his three-year-old horses was euthanised on the track.

Baffert headed to the winner's circle at Baltimore's Pimlico race course on the same day that his colt Havnameltdown went down with a fatal left leg injury in an undercard race.

"Winning this," Baffert said, choking back tears after National Treasure's win, "losing that horse earlier really hurts. It's been a very emotional day."

The fatality was another dark moment for a sport already reeling from the deaths of seven horses at Churchill Downs in a 10-day span leading up to the Kentucky Derby.

Derby winner Mage finished third in the Preakness after starting as the favourite. His defeat means there will not be a Triple Crown winner for a fifth consecutive year.

National Treasure held off Blazing Sevens down the stretch to win the 1 3/16-mile, $US1.65 million race by a head.

Jockey John Velazquez won the Preakness for the first time.

Baffert had a rollercoaster day in his return to Pimlico Race Course from a suspension that kept him from entering a horse in the Preakness last year.

The thrill of victories by National Treasure in the Preakness and Arabian Lion in an earlier stakes race contrasted with the agony of Havnameltdown's death.

"It's sickening," Baffert said. "We are so careful with all these horses, and it still happens. It is something that is disheartening. I feel so bad for that horse, and I just hope that (jockey Luis Saez) is OK."

Saez was conscious and transported to a local hospital for treatment. A team of veterinarians determined Havnameltdown's left front leg injury to be inoperable.

By evening, Baffert was celebrated for winning the Preakness for a record eighth time, breaking a tie with 19th-century trainer R. Wyndham Walden. 

This was Baffert's first Preakness in two years because of a ban stemming from 2021 Kentucky Derby winner Medina Spirit's failed drug test that led to a disqualification in that race.

A Hall of Famer and a longtime face of horse racing, Baffert sought to move past his suspension when asked Friday.

"We just keep on moving forward," he said. "We have other horses to worry about. A lot of it is noise, so you keep the noise out and continue working."

While horse racing deaths in the US are at their lowest level since they began being tracked in 2009, adding another at the track hosting a Triple Crown race will only intensify the internal and external scrutiny of the industry.

New national medication and doping rules are set to go into effect on Monday. The federally mandated Horseracing Integrity and Safety Authority, which already regulated racetrack safety and other measures, will oversee drug testing requirements for horses that should standardise the sport nationwide for the first time.