There were three gunmen wearing ski masks exchanging shots with Prince George's County police. There were five hostages. There was a long wait while tension mounted. And it was all make-believe.

The bullets were blank and the hostages were actors, helping the county police perform what has become an all too familiar assignment.

"This kind of practice really cuts down on error," said Harvey Goldstein, director of psychology for Prince George's police, who helps the officers resolve hostage situations. With the recent hostage incidents at Lake Braddock Secondary School in Fairfax County and the May shooting spree that left three dead at the IBM complex in Bethesda, none of the nearly two dozen officers in yesterday's exercise needed to be reminded how critical the training was.

In Prince George's County alone, police have had "116 barricades," as they call such incidents since 1976. In that time the officers have fired only once and that shot killed a Bradbury Heights man who was firing random shots in his neighborhood.

According to Capt. Robert Phillips, head of the county police Special Operations Division, mock barricade and hostage situations are held two are three times a year to review and evaluate procedures used by the force's Conflict Management Team.

Yesterday's exercise, held in a business district near the Forestville State Police Barracks, was termed a success by some police officials. Phillips, however, said the "one glaring mistake" occurred when the officers failed to search a rescued hostage. "She could have been one of the hostage-takers," Phillips said.

The Conflict Management Team, organized in 1976, is comprised of two 5-man emergency services teams (SWAT officers); communications equipment specialists, three scene commanders and 24 officers who are trained to negotiate with people holding hostages, or threatening suicide.

The roles of the gunmen and the hostages were played by a group known as "P.A.C.T." (Police Action Consultation and Training), a team of psychologists, actors and social workers. The team, employes of a private consulting firm, is headed by David F. Swink, a member of the psychodrama therapy and training unit at St. Elizabeths Hospital in the District.

"These initial 30 minutes are the most intense and most critical." said Phillips, as the exercise began in a vacant building.

For nearly four hours, two police negotiators talked to the suspects by telephone, talking deals and making arrangements for a doctor to be brought in to attend one of the robbers who had been wounded during the shootout with police. The hostages were released one by one, and finally, the "criminals" -- apparently convinced there was no escape -- surrendered.