A Montgomery County Juvenile Court judge ruled yesterday that Bethesda teenager Sherry Windt was responsible for stabbing her mother to death in a Bethesda apartment in October 1975.

The ruling by Judge Douglas H. Moore Jr. that the 19-year-old "was involved" in her mother's slaying is the equivalent of a verdict of guilty in a proceeding for an adult.

The verdict ended more than two years of legal and psychiatric debate over the mental state of the seemingly poised and sophisticated young woman, whom one psychiatrist once described as having multiple personalities that shift "right before your eyes."

After announcing his verdict in the nonjury proceeding, Moore accepted the joint recommendation of prosecutors and defense attorneys and ordered that Windt be sent back to the Baltimore-based psychiatric hospital where she has been receiving intensive treatment for the past year.

She is expected to be eligible for outpatient status at the Institute of Psychiatry and Human Behavior by the end of June.When Windt turns 21, the Juvenile Court will no longer have jurisdiction over her and she will be free to leave the institute.

Windt, looking frail and wan in a long flowered skirt and peasant top, stared straight ahead as Moore pronounced his finding of fact. "It seem like it's gone on forever,' Windt said later of the three-year-old proceedings against her. "I want to get on with the rest of my life," she added in a strong voice.

In the long series of hearings to determine first whether Windt was competent to stand trial and then whether she was sane at the time of her mother's death, psychiatrists held that the former Holton-Arms School student was extraordinarily bright, but severely emotionally disturbed.

During one of the first hearings on the case, Dr. Reginald S. Lourie of the National Institutes of Health, at Walter Reed Hospital testified that Windt suffered from split personalities, like the celebrated character of the movie, "The Three Faces of Eve."

At other points in her treatment, other psychiatrists have said that Windt was suffering from amnesia, and that she would fall into "dissociative states," in which she would lose contact with reality.

Moore said that psychiatrists at the institute believe the girl no longer is a threat to herself or to the community.

During the next two years, Windt's doctors will be required to keep in close contact with the court concerning the girl's treatment, whereabouts, and any decision to release her into the custody of a "responsible party" on an outpatient basis.

WIndt, a slender, pale young woman with straight brown hair that falls beneath her shoulders, said the therapy she has been receiving is "the best thing that ever happened to me." Darting her hand in the air frequently for emphasis, said the most important thing to her now is "a sound mind and a sound emotional structure."

Majorie Windt, 42, an advertising executive at Garfinckels, was found dead in the bedroom of her Bethesda apartment, with seven stab wounds over her neck, chest and side.

According to the police reports, in the weeks before her mother's death Windt told a classmate from the Holon Arms School that she was thinking her mother by poisoning the mother's tea.

Windt wanted her mother out of their apartment at 5101 River Rd. so that she could live there by herself, Windt's friend told police.

Windt told her friend she had thought of stabbing her mother to death, but later Windt dismissed that option as "to messy," according to the police report.

On Oct. 16 at 10:45 p.m. Windt called her psychiatrist, Dr. Robert Moran, and sobbing told him that her mother had committed suicide, according to the police report. Shortly afterward, Windt called her friend from Holton Arms and said "I did it," according to the report.

When police arrived at the family's "moderately furnished" apartment. Windt dressed in leotards, already had been sedated by Moran, according to defense attorney Walter Madden She had two fingernail marks on her right wrist, a scrape on her right forearm and a spot of blood on one elbow, the report said.

When asked if she knew the whereabouts of the knife that apparently used to kill her mother, Windt replied, "I took it out of her and washed it off," according to the report.

Windt told police that the blood around her elbow came from trying to clean the blood away from her mother, and that the scratches were really a case of hives and that she received the scrapes on her arm at school.

In his closing argument, defense attorney Madden asked rhetorically "Who killed Marjorie Windt? Was it Sherry Windt? Was it Dr. Moran? Was it Marjorie Windt herself?" He added that there was no proof beyond a reasonable doubt to show that Windt was the only person who could have killed Mrs. Windt.

Madden added that Windt had nothing to gain "from by killing her mother. Young girls don't go around stabbing their mothers . . . the last person the court would expect to carry out this act would be the daughter."

Assistant State's attorney Ann Harington, dismissed as "fancies" Madden theories that Mrs. Windt killed herself or was killed by someone other than her daughter.