A Great Falls woman who pleaded guilty last October to killing her two young daughters has been allowed to leave her psychiatric hospital on passes nearly every weekend since February, angering Fairfax County prosecutors who said they were not told of the arrangement.

Dr. David L. Wastos, a psychiatrist, said in a letter on file at Fairfax County Circuit Court that the releases are essential to the treatment of 31-year-old Nancy Lee Kantarian and that "the medical necessity of hospital confinement is diminishing."

Kantarian has been accompanied by Wastos or by family members during the releases from Sheppard and Enoch Pratt Hospital, a private, $300-a-day psychiatric hospital in Towson, Md., where she is incarcerated indefinitely.

Kantarian, the wife of prominent Washington attorney Harry K. Kantarian, pleaded guilty Oct. 5 to two counts of voluntary manslaughter in the May 21, 1984, deaths of her daughters. She stabbed 6-year-old Talia 32 times. Jamie Lee, 5, died of a blow to the head and burns from a fire at the Kantarians' $400,000 home, according to court papers.

Lawyers for Kantarian, who received a suspended 20-year prison sentence in the deaths, said at the time of sentencing it would be years before she was released. Nine examining psychiatrists said then she was extremely ill and would require years of intensive treatment.

Fairfax County Commonwealth's Attorney Robert F. Horan Jr. said yesterday he was "very unhappy with the notion that she is being released without any notice at all to this office, going as far back as February. I am not the least bit happy with the idea that everybody seems willing to accept the fact that there's been a miraculous cure here."

"Her normal treatment is a matter between her and her physician," said Albert J. Ahern Jr., one of Kantarian's lawyers. "I don't see that this is even an issue of concern at this time."

Fairfax Circuit Judge Barnard F. Jennings, who sentenced Kantarian, ordered that a hearing be held before Kantarian's release from the hospital. Horan said he thought that meant there should have been a hearing before the weekend passes were approved.

No such hearing was conducted, according to Horan. Wastos, the psychiatrist, declined to discuss the case.

In a May 24 letter to Judge Jennings, Horan said he had just learned of Kantarian's passes and of a proposal to leave all future passes to the discretion of her physician.

"I write this letter so that the record will reflect my strong objection to passes . . . without prior notification," Horan wrote. "I recognize that miraculous cures are not unusual in those who have been convicted of crimes."

The weekend passes were temporarily suspended after Horan's letter to Jennings, but were restored after a closed meeting July 8 in Jennings' chambers.

Kantarian's attorneys and her doctors say the weekend passes are a critical element in her recovery.

"For therapeutic reasons, I have . . . accompanied her to her house in Great Falls, and . . . to the cemetery in New York where the children are buried," Wastos said in the letter filed with the court. Kantarian has also been out with her husband and her family, Wastos said: "Her sign-outs have been instrumental in stimulating new material for psychotherapeutic work."