Fire caused $225,000 in damage to the amphitheater at the Merri- weather Post Pavilion in Columbia early yesterday morning, Howard County police reported.

County fire officials said that a police officer patrolling the area near the amphitheater at 12:35 a.m. saw flames shooting into the air and called the fire department. Capt. Raymond Faith of the county fire department said that 12 pieces of firefighting equipment and 50 firefighters from Howard and Montgomery counties responded to the alarm and extinguished the blaze by 2:30 a.m.

Fire investigators said they had not determined the cause of the fire, Faith said. Officials believe the fire began in the rear of the stage and spread to dressing rooms and up toward the roof, Faith said. Faith said that the fact that the wooden structure had been treated with fire-retardant chemicals had a significant impact on preventing the fire from spreading further. The amphitheater has seating for 5,000 people, plus lawn space for 5,000 more.

The fire will not delay the opening of Merriweather Post, which is scheduled for the second week in June, according to Charles Blum, spokesman for the Nederlander Organization, which books the shows at the center. The pavilion is owned by a subsidiary of the Rouse Co., which built much of Columbia. Blum said Rouse plans to have repairs made in time for the first show.

On April 4, 1982, a fire that burned for 17 hours destroyed the $5 million Filene Center at Wolf Trap Farm. Fairfax County fire officials blamed the fierceness of that fire on high winds that spread the flames along the wooden beams that had not been treated with fireproofing chemicals.