Mary Ann Cronin, a Navy nurse, said she was five minutes from her Fairfax City apartment, eager to get home after a nurses' reception, when she heard a thump, and then the windshield of her car shattered.

"I knew I hit something, somebody," the 34-year-old nurse said, sobbing, during her hour-long testimony in Fairfax Circuit Court yesterday. "I got out of the car and went back down the road and there was a boy on the ground. I thought: 'Please, God, don't let him be dead.' "

John Svec, 16, a sophomore and football player at Woodson High School in Fairfax County, died three days later of a skull fracture and brain hemorrhage.

Cronin was charged with driving while intoxicated and involuntary manslaughter and is on trial on the latter charge. A jury is scheduled to begin deliberations today after hearing two days of dramatic testimony.

Cronin, a lieutenant with a 12-year career in the Navy, testified she spent several hours on the evening of May 13 at a reception in Rockville commemorating the 75th anniversary of the Navy Nurses' Corps. Over six hours, she said, she drank six beers and ate about two dozen bite-size egg rolls and a bacon-cheeseburger.

Weary from the night's activities and a 10-hour shift in the surgical ward of Bethesda Naval Hospital, she said, she left about 11 p.m.

She testified she was thinking about the big reunion party scheduled for the next night and the old acquaintances she'd seen for the first time in years at that night's reception.

As she pulled off the Beltway and on to Little River Turnpike in Fairfax County, Cronin said, "I felt tired. I was thinking, 'I'm glad I'm almost home.' I smacked myself about 10 times on the side of my face" to keep awake.

As she continued her voice began to quiver. "I was driving and the next thing I knew I heard a thump, then I heard the glass on the windshield shatter."

Bursting into tears, Cronin said, "I had seen nothing."

The trial has been closely monitored by members of the antidrunk-driving group Mothers Against Drunk Driving (MADD) who, along with members of the Cronin and Svec families, have packed the small courtroom.

Three teen-agers who had been walking with Svec along Little River Turnpike the night of the accident testified Monday that Cronin's car veered off the main roadway and onto the paved apron where the four youths were walking.

Cronin, wearing a white Navy nurse's uniform, testified yesterday that she didn't feel intoxicated and didn't think she had swerved out of the driving lane.

"Is it possible you fell asleep and drifted into that lane?" asked Assistant Commonwealth's Attorney Jack A. Robbins.

"It's possible," Cronin replied softly. "But I don't think so."

Cronin said she asked the boys to help her pull Svec "out of the road so he wouldn't be in the traffic lane. I checked to see if he was breathing. I could see his chest rising and feel his breath. I was just terrified. I just held his head back so the airway would stay open."

Police officers testified that Cronin failed several sobriety tests at the scene of the accident. When defense attorney Barry Murphy asked Cronin to perform those same tests in the courtroom yesterday, she responded haltingly, quivering and losing her balance as she tried to walk a line heel-to-toe and stand on one leg.

After her testimony, Cronin walked unsteadily back to the defense table and collapsed in the arms of friends, sobbing.

A physician testifying for the defense yesterday said Cronin had not had enough alcohol to substantially impair her activities. Her attorney has argued that the boys were walking along an area of the road not designated for pedestrians.

Police records introduced at the trial show Cronin had a blood alcohol level of .14 shortly after the accident. Under Virginia law, a driver is considered intoxicated when the level reaches .10.