After months of would-she or wouldn't-she speculation, Jean Harris took the stand in her murder trial today and gave a breathless courtroom an intimate and detailed story of her relationship with Dr. Herman Tarnower.

She broke down and cried when she related her first meeting with Tarnower, 14 years before the shooting that took his life. She spoke of a proposal of marriage made, accepted and then taken back; she spoke of the doctor's other women.

She revealed Tarnower in the least flattering light to date -- a man who went to a dinner party while she stayed at his Harrison, N.Y., home and worked on what would be his best-selling book, "The Scarsdale Diet"; a man who showed his appreciation for that work with a check. (That form of thanks upset her. "'I don't want a pound of flesh,'" she wrote him, "'I was quite happy to settle for a thnak you.'")

Harris, the former headmistress of the Madeira School in McLean, Va., revealed, on the stand, much of herself as well. She showed herself as a woman who held herself in low esteem and was slow to anger -- when her lover called off the wedding, for example, she at first reacted with understanding. She also showed herself as a woman with a deadly and biting wit, whatever the circumstances. When Tarnower broke off their engagement, she sent back her diamond ring with a note suggesting he give it to his maid.

"'Give it to Suzanne,'" she quoted that note as saying. "'She's the only woman you'll ever really need.'"

Nor was she without wit when speaking of herself in her present circumstances. A report on Harris while she was at the Madeira School spoke of her as the "most controversial head of a private school in the country."

"I didn't think I was controversial," Harris said today, wryly. "I can't make that statement now."

Harris has been in court since October, charged with second-degree murder in the fatal shooting of Tarnower March 10, 1980, in the bedroom of his fashionable home.

The prosecution, noting that Tarnower had ben seeing another woman, claims Harris shot Tarnower in a jealous rage; the defense has called the shooting "accidental," though it has yet to explain how Tarnower was accidentally shot four times. Defense attorneys also had declined to say whether or not they would put their emotional and at times outspoken client on the stand. Harris, according to courtroom consensus, was simply to volatile and unstable.

But today the defense lawyers apparently determined to risk it. Harris was their only witness for the day and her testimony will continue Wednesday.

Harris, pale and struggling for composure, took the stand in a high-necked pale beige suit and weakly answered her lawyer's questions, going through the story of her life. She drew a sympathetic, helpless and charming picture of herself as a kindergarten teacher nervous about whether or not to take on the pressurs of teaching first grade ("'I said what do you do in first grade?' They said, 'Nothing, just teach them to read.'").

She was equally sympathetic as she explained why, after her divorce, she had decided to move from teaching to administration. "I couldn't really support us," she said, speaking of her role as a mother of two young sons and visibly gaining confidence on the stand.

But as defense attorney Joel Aurnou quickly moved her story to her first meeting with Tarnower, her confidence failed. Her voice choked, the muscles in her throat tensed. She put on her dark glasses, as she often does in moments of emotional stress, and broke down.

"It was December '66 . . .," she began, starting to cry and choking on her words, "at a party . . . he'd been to Russia and I'd been to Russia and we sort of showed off to each other how much we knew about Russia . . ."

She slowly regained her composure and moved through the happier days of the relationship. A first date in New York in March '67, when they went dancing at the Pierre -- "a place I've loved ever since." Tarnower, she added, in a characteristic note of self-deprecation, was "a better dancer, but I got to be better after I danced with him for a few years." She spoke of the first time she went to his home, the roses he sent her -- all during that happy time, "lots of roses" -- the calls that came at 6:30 each night.

In May 1967, Harris continued, Tarnower asked her to marry him and "gave me a lovely ring." But a few months later, when she pressed him on the date, he called off the wedding.

"He said, 'Jean, I can't go through with it . . . I'm afraid of it. . . .

I'm sorry,'" Harris testified.

Her response was perhaps indicative of a woman who supressed her anger. It also revealed a woman who knew, on some level, what sort of man she was involved with.

"I wasn't terribly surprised," she said. "I supposed that if he wasn't married by that time, he couldn't be . . . He was very sweet about it; he said all the right things, that we should see less of each other because I was a wonderful woman and deserved to be married. . . . I was very much in love with him, by then, being married didn't really matter. . . ."

The affair continued. Harris took back the engagement ring at Tarnower's insistence and she and the doctor traveled together, taking trips to the Caribbean, to Europe, around the world. It was assumed that their weekends would be spent together. "It wasn't a question of leaving him and wondering when I would see him again," testified Harris, "It was a question of what we were going to do the next weekend." They were, said Harris, "very close" during this period. "We saw each other every weekend, and it was not just geographic. I think it was because Hy had come to terms with reality -- with the fact that he was never going to marry anyone and he finally felt comfortable with that," said Harris.

However close they may have been, Tarnower apparently saw no reason to stop seeing other women, and Harris seemed to accept that. In 1969, Tarnower told Harris he was going to marry another woman, though he did not. On a trip to Europe in 1974, Harris saw very clearly that another woman was involved in Tarnower's life, and she told the story, under questioning from defense lawyer Aurnou, with considerable irony and wit.

"We went to our hotel room -- it was the Ritz, in Paris -- and when we opened the door there was a letter -- actually it looked more like a term paper -- from another woman," said Harris in an acerbic tone. "Hy [Tarnower] hid it or something, he looked a little uncomfortable with it. Then he took off his cufflinks -- he had a great gold cufflink collection -- and went into the bathroom to get ready for dinner, and I took off my earrings and I saw these cufflinks which I had seen for many years . . . he said, as he always did, they came from a grateful patient."

"And what did the underside of the cufflinks say?" asked Aurnou, leading his client along a path that obviously pleased him.

"They said, 'All her love, from Lynne Tryforos,'" said Harris.

Other sorry tales followed, not all of them dealing with Tarnower. Harris spoke at length of a report commissioned by the Madeira School that recommended she be dismissed immediately; she spoke of her growing feelings of unhappiness at the school. She said that even though a second report was "highly supportive" of her, it became apparent to her that she would have to leave the school, that she would "have to pack up her home and start again somewher else . . . . and it was very depressing and exhausting."

At the same time, the gentleman in her life, according to her testimony, was acting as he always had -- with the exception that a new career opportunity had come into his life. Tarnower had ben offered a contract to write a diet book.

Writing, however, apparently did not come easily to the doctor. Arriving at his home in the summer of '78, Harris found Tarnower "more upset" than she had ever seen him.

"He was so wonderfully in control of his life in everything he did, and it had gotten out of hand with the book," Harris testified. She helped him. She helped him a great deal.

"Some of the pages in my handwriting are in the book exactly as written," she said. He repaid her kindness with a terse note. "For reasons that I cannot explain, it is imperative I make all book disbursements at this time," his letter, read by Aurnou to the jury, said. "I am enclosing a check for $4,000 that I hope you accept."

She did not. She responded with a note of her own -- a note feminists would likely not admire.

"Having worked with you for a few days, doing something that really helped you for the first time in 11 years, ranks as high on the list of good memories as all those happy trips we took together," Aurnou read from her response. ". . . I was so honestly happy to be a useful part of your life. . . . I felt quite at peace while I worked and you went out to a dinner party. . . ."

Such "happiness" does not seem to have been part of her life for long. Harris was increasingly troubled with "exhaustion," and with a fear of not being able to handle her job because of that exhaustion. She took drugs, prescribed for her by Tarnower. And in the autumn of 1978, Harris testified, in a small voice, she bought a gun.

"I was frightened by a kind of exhaustion," she said, the exhaustion showing as she struggled to speak at the end of a long day on the stand. "I felt feelings of possibly not able to do my work any more. . . . I felt hopeless, I went out and bought a gun. . . ."

She sighed, swallowed and sighed again. "You had to wait a couple of days to get it and I didn't go back . . . . but later I did go back," she said in a tense voice. "It felt like a security blanket. . . . I felt safe with it. . . . I felt if I couldn't function anymore, I could handle it. . . . I didn't have to worry about feeling helpless. . . ."