The unmanned and unwanted ambulance sat at the curb yesterday, not in the Riverdale fire station a few yards away, a tangible sign of an intra-department dispute that has left several thousand Prince George's County residents with subpar emergency medical response since Sunday.

In a letter to the county three days ago, the volunteer firefighters of the Riverdale fire station, who work largely at night and on weekends while paid professionals work the day shifts, announced they no longer had enough members to respond to medical emergencies with their ambulance.

The county ordered them to continue, but the volunteers' ambulance then "vanishes," said Lt. Mark Brady, spokesman for the county fire department. "It's gone. We'd like to know where it is." County officials then dispatched a replacement ambulance, but the volunteers have refused to use or even bring it inside the fire station, Brady said.

Riverdale's paid firefighters have used the loaned ambulance to answer 911 calls during their 7 a.m. to 5 p.m. shifts. But the volunteers' refusal to respond to ambulance calls in the evenings and overnight has forced officials to call upon nearby stations to answer medical emergencies in the vicinity of the Riverdale station.

Response times can be slightly longer as a result, Brady said, although no life-threatening situations have occurred. County fire officials are waiting to see if the Riverdale volunteers heed demands to resume ambulance runs before choosing from among several options to ensure service, including increasing the number of professional staff available to respond.

"We have concerns," said Ann M. Ferguson, mayor of Riverdale Park, a town of 5,200 that relies on the Riverdale fire station more than any other.

In an odd blend of public and private firefighting that is characteristic of Prince George's County, the Riverdale building on 48th Avenue and most of its vehicles are owned by the Riverdale Fire Department, a volunteer group. But some other equipment is provided by the county, county firefighters staff the facility much of the day and county officials have overall command of operations.

At the station yesterday, Charles A. Ryan III, who was the volunteer chief of the Riverdale department until demoted this week because of the dispute, would not comment. Ryan said his membership had voted to discontinue ambulance service.

With fewer volunteers, Ryan wrote, "We feel that we can no longer provide the quality of a consistent and reliable response with the ambulance. Rather than remain in service and possibly create a further delay in the administration of emergency care that the citizens of the county value, we feel it would better serve them to cease operations."

Brady said, however, volunteer firefighters generally do not like to respond to medical emergencies, preferring to fight fires. "People join volunteer fire companies because they want to ride firetrucks," he said.

The dispute with Riverdale might have started three weeks ago, he said, when the county told Riverdale to loan a firetruck to the Clinton Fire Department. That department needed a truck to replace one that was out of service.

"Volunteer memberships are proud to display their apparatus," Brady said. "They were not willing to let that go."