Four persons were injured, one critically, yesterday afternoon when the driver of a cement-mixer lost control of his truck and collided with an ambulance at an intersection south of Fort Totten Park in Northeast, police said.

The accident happened at 12:16 p.m. when the truck's brakes failed while the truck was traveling south on Fort Totten Drive, down a hill, authorities said. The truck's driver, Walter Manning, swerved to avoid another truck, which was stopped at Fort Totten Drive and Hawaii Avenue.

Emergency personnel in an ambulance heading west on Hawaii spotted the out-of-control truck, and the ambulance driver put his vehicle into reverse to avoid the truck's path, police said. One emergency technician jumped from the cab of the ambulance. But the truck slammed into the ambulance and ended up flipped on its side on a nearby embankment.

Ambulance driver Douglas Frazier was in critical condition at Washington Hospital Center with head and chest injuries, and his colleague, Ricardo Clark, was in fair condition with back strain, according to hospital personnel. M. Wayne Cobb, another emergency technician, also was admitted to the hospital, and was listed in good condition last night with severe bruising.

Manning, who works for Opportunity Concrete Corp., was pinned inside the overturned truck for about 15 minutes before emergency personnel freed him. Hospital officials said Manning suffered injuries to his head and chest, and was in fair condition.

About four hours later, three other emergency workers were injured in a collision with a vehicle about a mile away at North Capitol Street and Michigan Avenue NW, fire officials said. The ambulance workers were taken to the same hospital, as was the driver of the other vehicle. Other details could not be learned immediately.