After eight days of deliberation, a jury this evening found Jean Harris guilty of the second-degree murder of her lover, Scarsdale diet doctor Herman Tarnower. The verdict carries with it a mandatory prison term of 15 years to life.

Harris did not respond as the verdict came.

Pale and composed, with no member of her family in the courtroom, the former headmistress of the exclusive Madeira School in McLean, Va., sat impassively as she heard the verdict, her hair constrained neatly under her signature headband, her face a fixed mask.

That mask remained as she heard the foreman of the jury declare her guilty of additional charges of second- and third-degree criminal possession of a weapon. Her face remained still as she was led from the courtroom by a woman marshal who took her gently by the arm.

A mob of reporters who packed the courthouse basement last saw her in the back of a marked police car, a woman marshal beside her, behind a cage.

A second police car led the way as Harris was taken to Westchester County Jail at Valhalla, her face still a mask.

Comments to reporters, who numbered more than 100, were left to her attorney.

"She's an intellectual and intellectually knew it could happen, as I did," defense lawyer Joel Aurnou told reporters as his client was taken away. "But I don't think she, any more than I, thought it would happen."

The jury did not believe Harris' testimony and based its decision on that disbelief, according to juror Marie Jackson of New Rochelle, N.Y. The jury's first vote was 8 to 4 in favor of conviction, Jackson said.

The dramatic culmination of one of the nation's most publicized crimes of passion, the verdict in the trial came at precisely 6 p.m., after deliberations that took nearly 48 hours over eight days.

The atmosphere was tense, the courtroom was jammed. Onlookers had come to regard the Harris trial as a sort of soap opera, better than theater because it was life.

That atmosphere had heightened as the judge, Russell R. Leggett, cautioned the news media that "whatever the verdict," he "didn't want any outburst whatever."

Harris, in a fawn suede jacket and high-necked blouse, was already seated as the judge made his announcement. She was flanked by Aurnou, and, to her left, three additional members of the defense team. Neither her Marine son, Jim, nor her elder son, David, a local banker, were with her.

Harris, as she would be throughout the 10-minute proceeding, was perfectly composed. Later, under questioning from reporters, Aurnou would admit that his client "was on medication, right up to the trial and at the time of the verdict."

It was not, however, apparent at the verdict. Harris sat, obviously tense, but alert, occasionally whispering to her attorney, who looked, as he has in the last few days of the these proceedings, tense and worried.

The prosecutors, including the Westchester County district attorney, came in, then the jury of eight women and four men. Their faces were hard. Many averted their eyes when facing the defendant.

The judge called on the foreman of the jury, Russell Von Glahn, a bus mechanic from New York.

"I understand the jury has reached a verdict?"

"Yes, we have," answered Von Glahan, a middle-aged man with the profile of a lumberjack.

"And is the verdict unanimous?"

"Yes, it is."

The judge's clerk, Thomasina Cook, then read out the charges, and the foreman's answers rang through the courthouse, a hard litany:

"On the charge of murder in the second degree, how do you find?"

"Guilty."

"On the change of criminal possession in the second degree, how do you find?"

"Guilty."

"On the charge of criminal possession in the third degree, how do you find?"

"Guilty," the foreman said.

Every eye in the room was on Harris, but she did not move. Nor did she respond as the judge allowed a proceeding that occurs only in a case where the deliberation, as in this trial, has been long and difficult -- the practice of asking every juror, one by one, to call out his verdict.

"Russell Von Glahn," the court clerk called out, then, "Geneva J. Tyler. . . ." and one after another the voices responded, "Guilty," "Guilty," "Guilty. . . ."

Harris turned to her attorney. "I can't sit in jail," she whispered.

There was little else. The judge thanked the jury for their "service," saying "No one can say this wasn't a fair trial." He told the jurors that it was the desire of the top administrative judge of the county, Joseph Gagliardi, that they not discuss their deliberations with anyone, even though their work was over.

"To disclose what took place would be a breach of confidentiality between you and your colleagues," he said. "It could also serve as a deterrent to future jurors."

A date was set for Harris' sentencing -- Friday, March 21, a year and 11 days after she shot the doctor to death.

And the prosecutor, George Bolen, asked quietly that Harris be "remanded" -- taken into custody.

"As is mandatory," the prosecutor said.

A few minutes later, the gray-uniformed woman sheriff who has been in this courtroom since the beginning walked behind Harris and put a and on her shoulder, and Harris, the expression still fixed, walked out of the courtroom into custody.

Harris' attorney said he will appeal. No bail can be set until the sentencing.

As Harris left the courtroom, members of the Weschester district attorney's office crowded around Bolen and pumped his hand vigorously. Bolen broke into a wide grin.

"We always had faith in our case, and the jury came in with the right verdict," he said.

Harris had been in court since October, charged with the second-degree murder of Tarnower in the bedroom of his Westchester home last March.

Second-degree murder is the most serious charge possible in New York state and requires that the jury believe the murder was intended. First-degree murder in New York is limited to the killing of a police officer and is punishable by death, but the state has no death penalty.

With Tarnower, the author of the best-selling Scarsdale diet book, and Harris, the headmistress of a prestigious school, the case immediately made headlines.

The defense strategy made headlines as well. Rather than plead temporary insanity, or guilty, the defense maintained that Tarnover had been killed in a "tragic accident" -- even though the doctor sustained four wounds.

The Harris defense team also claimed that Harris had driven from her home in Virginia to the doctor's home, not to kill Tarnower but to kill herself.

Asked if the jury believed Harris planned her own suicide, juror Jackson said, "When she was driving there, maybe she did, but it didn't work out that way. She got mad and, when you take out a gun and point it at someone, that's intent."

The defense's position seemed impossible, particularly because Harris had made several incriminating remarks to police on the night of the shooting.

"He's slept with every woman he could and I'd had it," Harris had said. She had also, as police were struggling to carry the doctor to an ambulance, asked a question that didn't seem to reflect to concerns of a woman who had come to her lover's home to die. "Who did he have for dinner?" she asked. h

Pretrial hearings on the case lasted a month, jury selection took nearly another month, testimony took 12 weeks. And as the Jean Harris story emerged, the case gained momentum. It was not simply that the principals were wealthy or socially well-attached (Tarnower counted among his friends Iphigene Sulzberger, of The New York Times family, and a Canadian ambassador at large), it was that the trial contained so many other soap-opera components.

Harris was not merely a Headmistress, but a Slim Blond Headmistress. Tarnower was not simply a Diet Doc, but a Best-Selling Diet Doc. The details of their affair were the stuff of pulp fiction.

There was Another Woman -- the doctor's assistant, Lynne Tryforcs, dubbed by columnist Murray Kempton "the Lorelei of Larchmont." There was Womanizing on the part of the doctor throughout the 14 years of his affair with Harris, descriptions of expensive vacations around the world, hints of drug abuse. And the fact that Harris, 57 at the time of the shooting, was apparently discarded by the doctor in favor of a woman of 37, gave an extra dimension to the case: It made Harris a cause.

Disgarded she might have been; still, Tarnower left her $220,000 in his will -- but she will not be able to inherit it. A New York law prohibits a criminal from benefiting financially from his crime.

The trial drew 91 witness, but an air of drama was added as no one knew, until the day she appeared, whether or not Harris would take the stand. She did, for eight days. Sometimes sobbing, sometimes laughing, she was an effective witness as she told the jury of her affair with Tarnower and also of the depressions that had plagued her for the last 10 years and culminated the week before the shooting.

"I had always felt inadequate, but that week, I was terrified by my inadequacy," she said. She claimed to have never been threatened by her lover, and impressed the courtroom with her wit on the subject -- as when her rival took an ad in The New York Times to proclaim her love. "Why doesn't she rent The Goodyear Blimp next year?" she asked, "I hear it's available."

But her credibility -- and the defense position -- suffered its greatest blow when The Scarsdale Letter, a letter she had written Tarnower the day of the shooting, came into evidence. The defense had tried to keep the letter out of court for 11 months, Harris herself had dismissed it as "a whining letter." But, as it became public, it was clearly the letter of a furious woman, enraged that her position in a man's life had been usurped.

"I am distraught as I write this," she began, "your phone call to tell me you prefer the company of a vicious, adulterous psychotic . . . has kept me awake for 36 hours . . . what I say will ramble but it will be the truth . . . aand I have to do something besides shriek with pain. . . ."