Navy nurse Lt. Cmdr. Mary Ann Cronin was acquitted yesterday of involuntary manslaughter in the death of a 16-year-old Fairfax County youth whom she struck with her car last spring while driving home from a Nurse Corps reunion where she had consumed six beers.

Cronin buried her face in her hands and sobbed loudly as the Fairfax Circuit Court clerk read the verdict before a packed courtroom. The parents of young John Svec--described by the prosecutor as the "consummate blameless victim out with friends on a harmless lark"--appeared stunned. Friends and families of both Cronin and Svec gasped as the verdict was announced and wept as they filed out of the small courtroom, the scene of two days of emotional testimony in a case that has received extensive public attention.

Svec and three teen-aged friends were walking along the edge of Little River Turnpike near Northern Virginia Community College in Annandale about 11 p.m. on May 13 when Svec was hit by the car. The four youths were clad in military camouflage outfits at the time.

After deliberating more than six hours Wednesday, the eight-man, four-woman jury had reconvened only 25 minutes yesterday morning before delivering its verdict. The jurors were divided 11 to 1 in favor of acquittal when they left the jury room about 6 p.m. Wednesday, said juror Ron Petersen. The juror who was "holding out for guilty" returned to the jury room and voted for acquittal yesterday morning, Petersen said.

"We felt the evidence was conflicting in terms of her Cronin being impaired by the alcohol ," Petersen said. "We felt she was negligent, but not criminally negligent."

Cronin's lawyer, Barry Murphy, said that neither he nor Cronin would have any comment. Svec family members said they were shocked.

"There'll be other people dying as a result of this decision," said the youth's father, Robert Svec, adding that he believed the verdict indicates to drunk drivers that they aren't subject to punishment from the judicial system. "Anytime those people jurors read about a death caused by a drunk driver, they should know they have blood on their hands."

"I'm convinced the evidence was there for a conviction," said Assistant Commonwealth's Attorney Jack A. Robbins. "But this was not your usual criminal prosecution. Mary Ann Cronin is not your classic criminal." Robbins said he believed the jury was "looking away from the evidence and looking at the emotional and sympathetic aspects" of the case.

Cronin will be tried Sept. 21 on a separate charge of driving while intoxicated. Police records introduced during the involuntary manslaughter trial showed Cronin had a blood alcohol level of .14 percent when tested shortly after the accident. Under Virginia law, a driver is considered intoxicated when the level reaches .10. If convicted of driving while intoxicated she could be sentenced to 12 months in jail and/or fined a maximum of $1,000.

Robbins said Cronin was tried on the involuntary manslaughter charges first because Virginia law would not allow the prosecution to enter evidence of intoxication in the manslaughter trial if she had been found innocent of driving while intoxicated. The finding in the involuntary manslaughter trial, however, has no legal effect on evidence that can be introduced in the trial on charges of driving while intoxicated.

Throughout the two-day trial, both the prosecutor and the defense attorney repeatedly described the case as one that elicited sympathy for both families. Cronin, 34, a head surgical nurse at Bethesda Naval Hospital with a 12-year career in the military, is a "responsible person, respected in her field, who while driving home was involved in an accident," Robbins said in closing arguments.

"But the bottom line," he added, "is John Svec lying on the pavement with fatal injuries."

Cronin, remaining composed during her testimony until she began recounting details of the accident, said she never saw the four youths walking near the edge of the highway in a paved bus stop lane.

"Think of yourself driving down a roadway on a dark night in which people dressed in night fatigue uniforms are walking," defense attorney Murphy asked the jury in his closing arguments on Wednesday. "Think what position you'd find yourself in."

Juror Petersen said it was Judge Quinlan H. Hancock's 11 pages of instructions that eventually led the jurors to acquit Cronin. He said most of the jurors were prepared to convict her when they first retired to the jury room soon after 11 a.m. Wednesday.

But as the jury began wading through the judge's instructions, they began changing their minds, one by one, Petersen said. The jurors believed she had been drinking, but did not believe she was substantially impaired by the alcohol because she administered first aid to Svec after the accident. In addition, while awaiting her alcohol breath test at the police station, she coherently discussed one of the policeman's medical problems with him, Petersen said.

The jurors also did not believe she had been grossly and criminally negligent in the accident, a second factor that would have been required for an involuntary manslaughter conviction, Petersen said.

Representatives of the antidrunk-driving group, Mothers Against Drunk Driving, who had monitored the trial, said they considered the verdict a setback.