BALTIMORE, NOV. 20 -- Arson is suspected in a fire at a Pimlico stable that injured one racetrack security guard and one horse and caused an estimated $5,000 in damage Monday night.

Capt. Dennis Howell, an investigator for the Baltimore Arson Squad, said today the fire appeared deliberately set in four areas after hay and straw were spread across an area of the shedrow on the east side of Barn 10, near West Belvedere Avenue. The fire began about 9:30 p.m. in the brick-and-cinderblock stable and was brought under control by city firefighters within a half-hour. It was limited to a small area at the end of the shedrow.

Twenty-five horses were evacuated by track security personnel and backstretch workers and brought to nearby stables. Randolph Scott, a track security guard, was treated for smoke inhalation at Sinai Hospital and released.

Trainer Bill Albright said an unnamed 2-year-old filly of his suffered smoke inhalation and was in guarded condition with possible respiratory damage.

"She's been breathing real hard," Albright said. "The vets have been treating her since it happened, and we've contacted New Bolton {medical center at University of Pennsylvania} to make sure everybody's thoughts are the same."

Albright, whose 20 horses share Barn 10 with those trained by John Secor, hailed rescue efforts. "We were very fortunate; it could have been a lot worse," he said. "You've got to give lots of credit to the people who live on the racetrack and the guards. It was good teamwork. That saved it, really."

Albright withdrew his only horse entered today at Laurel because of possible effects from the fire.

Most of the horses had returned to Barn 10 this morning even as the smell of soot predominated. Walls and shedrow dirt were blackened, but the barn showed no structural damage, officials said. The stable, built in 1969, withstood a fire that probably would have devoured older wooden structures.Wolfendale on a Roll

Trainer Howard Wolfendale continued his remarkable success today at Laurel as Bold Jag became his 20th winner from 43 starters this meeting, a 46.5 winning percentage..