A Silver Spring psychiatrist, accused of medical malpractice for having an affair with a former patient, testified in Montgomery County Circuit Court yesterday that the woman, clad in a see-through nightgown, seduced him a month after her psychotherapy ended.

In a near monotone, Dr. Gerald H. Fink, 53, detailed his 2 1/2-year affair with Frances M. Dawson, a 44-year-old corporate secretary. He portrayed his former patient as a willful and manipulative woman who often threatened and humiliated him to get her way.

Dawson told a Maryland Health Claims Arbitration panel last year that her affair with Fink ruined her mental health and caused her to be hospitalized four times, and the panel ordered Fink to pay her $374,966 in compensatory and punitive damages. Fink appealed and is asking the jury of nine women and three men to vacate that award, maintaining that Dawson's lawsuit arose from a broken heart rather than a violation of medical ethics.

Dawson, who often cried and raised her voice during questioning yesterday by Fink's attorney, Steven R. Migdal, testified that she panicked when Fink asked her in August 1982 to move out of a house they had shared for nine months. "Dr. Fink had given me my mind and to lose him was to lose that," she said.

Fink denied Dawson's allegations that six months after she began therapy with him to help her gain independence from a domineering husband who was being sent to prison on a gambling conviction, he began kissing and petting her during therapy sessions in his office.

On Sept. 30, 1980, after Dawson told Fink she wanted to end her therapy because "her life was going well," Fink testified that Dawson "gave me a hug, a kiss on the cheek and I hugged her back."

Three weeks later, Dawson called and asked to see him, saying she had "a surprise . . . that had nothing to do with therapy," Fink testified. She arranged to come to Fink's Silver Spring home where he also maintained his office on Oct. 25, 1980, excused herself to go to the bathroom and emerged in "a see-through nightgown," and kissed him, he testified. "I reacted as a man," Fink said. "We were both excited and passionate and we made love."

The couple began meeting weekly or every other week at the office and started dating openly, Fink said. "By the summer of 1981, we were very much in love with each other and talked about getting married," Fink said. "As a couple, we were ecstatic."

Fink testified that he gave Dawson a diamond "pre-engagement ring" on her birthday on Nov. 14, 1981, to symbolize "my intention to love her and to marry her."

Dawson moved in with Fink in January 1982, Fink testified, after he separated from his wife. Fink said he and Dawson sometimes argued over money matters and the depth of his commitment. He said Dawson once humiliated him in front of his daughter, cousin and nephew visiting from out of town by making Fink sleep with his nephew instead of with her because she was angry that he would not come to bed when she wanted him to.

Before resting Dawson's case, her attorney, Henry E. Weil, entered into evidence doctor and hospital bills totaling more than $20,000 for psychiatric care for depression and suicidal tendencies that Dawson has received since Fink broke up with her in March 1983. Weil also submitted charges from Fink totaling $4,880 for psychotherapy sessions from September 1979 through September 1980.

Fink's wife, Renee, filed for divorce on April 11, 1985, according to court records.

Fink's testimony is expected to continue today before Judge DeLawrence Beard.