In 1972, Larry Smith built a plywood box in which to lock his hyperactive 7-year-old son during class in hope of convincing reluctant Fairfax County officials that the first grader could remain in public school.

He didn't succeed immediately. Kenneth Smith, who is mentally retarded and emotionally disturbed, was sent to a private school that year.

But Fairfax County began special classes for emotionally disturbed children the following year, and Kenneth attended public school until he was 12, when he was again placed in private school.

Kenneth Smith lived at home until 1983, when his parents could no longer cope with his belligerent behavior -- including outbursts, Mary Louise Smith said, that did thousands of dollars in damage to their house. Since then, he has lived in four different state institutions.

Now, the Fairfax couple have just lost another battle with the government, this one to prevent a state mental hospital from treating their son with drugs they contend could kill him. Last week, a Fairfax Circuit Court judge awarded medical guardianship of Kenneth to Western State Hospital in Staunton, which means state physicians may administer drugs without the Smiths' permission.

The Smiths want Kenneth treated with a unique nutritional therapy developed with a neurologist's help. The Virginia Department of Mental Health and Mental Retardation believes there is no medical evidence the nutritional therapy works, said David R. Hathcock, spokesman for the attorney general's office.

"I'm more scared for him than I am bitter about the system," Mary Louise Smith said. The Smiths have not decided whether to appeal. If they do not, she said they will "fight to change laws, see if we can get a better network for research" into children like their son.

Pursuing their son's case through the years already has meant many hours of studying biochemistry, meeting with lawyers and driving hundreds of miles to visit him. Their other child, a daughter, decided to become a nutritionist partly because of her brother's case, the Smiths said.

Kenneth turns 20 this week. The Smiths traveled 150 miles to Staunton during the weekend and brought him a cherry pie, baked to accommodate his diabetes, in celebration of his birthday.