A mentally retarded 11-year-old boy accidentally hanged himself early yesterday on the metal guardrails of his bed at a Fairfax County training center for the profoundly and severly retarted.

The boy, identified only as Michael, was found unconscious at 2:15 a.m. by an aide at the Northern Virginia Training Center, 9901 Braddock Rd. Following resuscitation efforts, he was pronounced dead an hour later at Commonwealth Doctors Hospital.

The boy's death was described by both Dr. David Lawson, director of the state-run facility, and Fairfax County police as the consequence of "a tragic and freak accident."

Michael's bed was equipped with three guardrails, about six inches apart, to prevent falling out of bed. Lawson said the boy's torso apparently slipped through the second and lowest rails but his head became wedged and he strangled.

He said that the profoundly retarded boy was so spastic -- a condition in which muscles contract in paralysis -- that he was unable to straigten his legs.

"If he could have straightened his legs, they would have easily touched the ground and he wouldn't have died, Lawson said."He apparently didn't even cry out. There were nurses just down the hallway from him."

Aides check bedrooms of children at the facility on the half hour, he said, and at 1:45 a.m. the boy and his three profoundly retarded roommates were sleeping soundly.

Most of the profoundly retarded children, who are unable to feed or clothe themselves or converse with others, have a foam pad on their bed rails to prevent them from bumping their heads during the night.

Michael didn't have one, Lawson said. "He was a sound sleeper and the kids don't like the pads anyway. They make them feel more confined."

"The police and I agreed it was a fluke. Nothing like this has happened before. We might change the rails, make more frequent checks. We just don't know right now," the director said.

Michale came to the center from the Lynchburg Training School in June 1978. About 56 percent of the 285 patients at the Northern Virginia center came from the Lynchburg school, an overcrowded, turn-of-the-century facility, Lawson said. He said the rest of the patients are from direct admissions from families and from referrals by local mental health retardation services boards.

The seven-year-old Northern Virginia center was the first of three centers in the state designed to provide intensive therapy for the mentally retarded.