Arson was blamed for a fire that destroyed two cars parked in a warehouse of a commercial cleaning concern in Rockville early yesterday, according to Lt. Ray Mulhall, a spokesman for the Montgomery County Fire Department.

"Somebody came in and torched those vehicles," said Ken Costella, a partner in the William F. Hurley Co. "If the sprinkler hadn't gone on, the entire warehouse would have exploded."

The Montgomery County Fire Department reported that of 3,108 fire calls answered so far this year, 974 have been attributed to arson.

Costella said nothing was stolen from either the warehouse or the adjoining office, located at 687-J Lofstrand Lane. In addition to the cars, a 1982 Blazer and a 1978 Buick, Costella said about $50,000 damage occurred to contents of the warehouse flooded by water from the automatic sprinkler system. Adjoining warehouses also suffered water damage, according to the fire department.

Firefighters said they were alerted to the scene by an employe of a neighboring concern, who reported water flowing out of the Hurley warehouse in the industrial section of Rockville.

Police said someone apparently entered the warehouse between 4 p.m. Sunday and 8 a.m. yesterday, when the fire was discovered. The warehouse did not have a burglar alarm.