"Congrats Gal suffered death after the eighth race," the Stronach Group, which operates the track, said in a statement.
"The incident occurred after the wire (finish). Veterinarians attended to the horse immediately."
A cause of death has not been determined and a full necropsy will be performed, the Stronach Group said.
The death comes after 23 horses suffered fatal injuries at Santa Anita Park in California between December and March, resulting in the temporary closing of the track.
People for the Ethical Treatment of Animals (PETA) immediately called for an investigation of the Maryland death.
"The sickening collapse and sudden death of Congrats Gal at Pimlico today are proof that the Maryland racing industry has not done enough to protect horses," the statement said.
"PETA will be calling for a formal investigation, including a necropsy and blood toxicology panel, and for the release of all medication and veterinary records.
"We will be contacting the district attorney's office, as we did in California, where the D.A. has appointed a task force to investigate training and veterinary practices."
Reuters has contacted the Pimlico track operators for comment.
The Preakness Stakes is the second leg of U.S. thoroughbred racing's Triple Crown. The first, the Kentucky Derby, was held on May 4 and the concluding race, the Belmont Stakes is set for June 8.
(Reporting by Gene Cherry in Raleigh, North Carolina; Editing by Toby Davis)
