A trembling Jean S. Harris, her face contorted with barely controlled rage, caused a stir in Westchester County courthouse today when she stormed out of her murder trial, shouted at one of her lawyers and regaled reporters with a stream of invective against a prosecution witness.

"This was the place for the truth to come out but I discovered it's not going to come out, so I'm going to start talking to the press," she told reporters in the court corridor shortly after the outburst, which occurred out of the jury's presence.

Harris, who is charged with second-degree murder in the March 10 shooting of Dr. Herman Tarnower, returned to the courtroom a few minutes after walking out. Her anger was provoked by Judge Russell R. Leggett's decision not to allow her attorney to introduce pretrial testimony by Detective Authur Siciliano concerning the sincerity of remarks she made to him the night Tarnower died at his Harrison, N.Y., home.

Harris, 57, former headmistress of the Madeira School in McLean, Va., previously had behaved more like the hostess at a garden party than a defendant in a murder case. She smiles often in court, chats with friends and reporters and appears to enjoy herself as much as anyone when the testimony takes a lighter turn. But events today, the 12th day of testimony, apparently got to her.

The prosecution, noting that Tarnower, the 69-year-old author of the Scarsdale Diet, had been seeing other women -- in particular, his assistant, Lynne Tryforos, 37 -- has tried to show that Harris shot her lover of 14 years "deliberately, consciously" in a jealous rage. The defense has maintained she intended to commit suicide and that Tarnower was killed accidentally in a struggle over the gun.

In his cross-examination of Siciliano today, defense attorney Joel Aurnou, referring to a conversaton Harris and the detective had the night of the killing, asked: "Did she tell you, 'I wanted to die. Why should he die?'"

"Yes, sir," Siciliano said.

"Did she tell you that several of the shots had been fired during a struggle?"

"Yes, sir."

"Did she appear . . . to be answering you sincerely?"

"I assume she was."

But when prosecutor George Bolen asked the officer whether Harris, in her statements and actions that night, ever appeared "to be acting," Siciliano replied in the affirmative.

Aurnou then tried to introduce portios of Siciliano's grand jury testimony that he hoped would show the witness was contradicting himself, but Leggett refused him and called a recess.

Harris grabbed her coat and stormed out of the courtroom. She soon returned and complained bitterly to reporters that "we have a sworn statement [Siciliano's pretrial testimony] which we were not permitted to make public. But I'm going to make it public later on."

She then told one of her attorneys to "shut up" when he apparently admonished her for the comments to the press.

In a press conference later, Aurnou said Harris was "hurt and offended at his [Siciliano's] attempt to change his testimony. It's outrageous for him to change his testimony now, nine months later."

Harris, still shaken, read from papers she identified as a copy of Siciliano's pretrial testimony. "'She was very sincere,'" she quoted him as saying Oct. 9. "'I asked her who owned the gun'" -- this reportedly from his Oct. 14 testimony. "'She told me it was hers.'"

"'She answered you sincerely?'" Harris went on, now quoting Bolen. "'She answered me sincerely,'" she said Siciliano replied.

"That was not permitted to be read for the jury," Harris said. "When that happens again and the jury is not allowed to hear the truth, I'm going to come out here and read from this script."

Later, Aurnou told reporters Harris' "only reason for being alive is to prove that she didn't kill Dr. Tarnower."