The death of 15-year-old Stephen M. J. O'Donnell, whose body was found Tuesday hidden under shrubbery of a neighbor's house in Fairfax County, was caused by alcohol poisoning, according to a preliminary medical report released yesterday.

Fairfax County Commonwealth's Attorney Robert F. Horan Jr. said O'Donnell died from consuming a large quantity of alcohol. The youth voluntarily drank several eight-ounce glasses of liquor at the home of some new friends Friday following an initiation rite into "membership" in a makeshift fort near Ft. Belvoir.

When O'Donnell lost consciousness and his friends were unable to revive him they panicked, according to police, and hid him first in a storage shed, later moving his body to the shrubbery in a neighboring yard.

Horan said the amount of alcohol O'Donnell consumed "might be lethal with most adults." He said the youth's blood contained .36 per cent alcoholand his urine .43 per cent. "You're a drunk driver at .10 (blood alcohol level.)" he said.

He said the toxicology report did not determine the exact time of O'Donnell's death, although other evidence shows that the Mount Vernon High School student "died some time during that first night (last Friday.)"

Horan said a decision whether to bring criminal charges against any of the youngsters involved in the incident will await a final medical report and additional interviews.

According to police, last Friday O'Donnell and two 15-year-old eighborhood youths went to the makeshift fort in a wooded area on the Ft. Belvoir Army post about a mile and a half from their homes.

O'Donnell rejected his friends' suggestion for an initiation rite that included a beer-drinking stunt, police said.

Later, when the youths returned to the home of one of them, O'Donnell said he wanted to drink hard liquor and a 13-year-old brother of one of the 15-year-olds pound vodka and scotch for O'Donnell from their father's basement bar, according to police.

After three hours, O'Donnell passed out and the two brothers who live in the house were unable to revivie him, police said. The boys wrapped O'Donnell in a blanket and carried him to a storage shed behind their house where they left him until the predawn hours Monday.

Then, police said, they carried the body across the street and hid it in shrubbery beside a neighbor's house where police found it Tuesday night.