Maryland stewards are to hold a hearing today on allegations trainer King T. Leatherbury's Flush N Diamonds, winner of the Dec. 21 ninth race at Laurel, had been given a prohibited drug.

Urinalyses by state and independent laboratories showed the presence of the prohibited drug theobromine. Leatherbury said at the time that his horse had been given vitamin supplements that contain cocoa flavoring, a property of theobromine.

Leatherbury, twice the national champion trainer, is leading the Bowie meeting with 26 victories.

Bowie General Manager Al Karwacki is seeking state general assembly legislation that would bar Timonium from racing 42 days beginning July 27. Karwacki and other mile-track administrators fear the influx of horses will force them out of stables at Timonium they had been able to use in the past. Horses had been shifted to Delaware Park, but it shut down. Insiders say it is possible an arrangement could allow Timonium to race at Pimlico or Laurel and share in the revenues.

Apprentice jockey Alberto Delgado, to be honored at the Federico Tesio awards dinner Feb. 25 as the outstanding apprentice on the Maryland circuit last year, faces a challenge to his 1983 lead at Bowie when he returns to riding Monday. While Delgado has been serving a seven-day suspension for a riding infraction, Donald Miller has been gaining ground and is only six behind Delgado's total of 47.