The prosecutor charged that the shooting was done in cold blood. The defense attorney pleaded that his client was temporarily insane.

And the judge, presiding over one of the most sensational trials in recent memory in the lush horse country of Loudoun County, warned the jurors against succumbing to a traditional view that he referred to as an "unwritten law."

"Does any member feel a husband separated from his wife is justified in attempting to kill his wife if he finds her making love to another man?" asked Loudoun Circuit Court Judge Carleton Penn yesterday at the start of the Leesburg trial of Middleburg horse trainer Theodore Gregory. "Do any of you believe in the so-called unwritten law?" Each of the jurors empaneled yesterday answered "no," the desired reply.

Gregory, 28, is being tried on three felony charges connected with a shooting incident last August that left his estranged wife, Monique Gregory, slashed on both arms and her lover, Howard LaBove, shot dead.

The Gregorys, who had been separated for four months when the incident occurred, and LaBove, 30, were all horse trainers and well known in the moneyed Middleburg horse set. The violence of that affair has shocked and intrigued that community.

"The sidewalks are talking," said one resident of Middleburg, renowned as the hub of a region of estates populated by millionaires.

At a preliminary hearing, Mrs. Gregory testified that her husband had crashed open LaBove's bedroom door while she and LaBove were in bed making love.

"He stood at the door and said, 'I'm going to kill you, b----,"' the 26-year-old woman testified then. "I was as good as dead."

According to her testimony, she escaped when LaBove pushed her aside and charged Gregory after he had fired one shot. Mrs. Gregory said she fled the house when LaBove, after being shot three times, pinned her husband against a couch. She ran to a nearby tenant house on the same farm a few miles outside of Middleburg.

While she was running, prosecutors said they will try to prove, Gregory emerged from the house and fired two shots at her. Upon reaching the tenant house, Mrs. Gregory plunged both arms through the window of a locked door; 300 stitches were needed to close her wounds.

LaBove was apparently dead when Loudoun County sheriff's deputies arrived half an hour later. Theodore Gregory was arrested the following morning at the Middleburg training center where he stabled horses.

Gregory's defense attorney, Blair Howard of Alexandria, said in his opening argument yesterday his client is innocent because he was the victim of "irresistible impulse."

Howard described Gregory as a man devastated by his separation from his wife and further upset by his wife's public appearances with LaBove. He said he would present defense witnesses to chronicle a pattern of "emotional despondency and deterioration" that he maintains led to the shooting on the night of Aug. 20.

"He was acting on impulse," Howard said. "When he [Gregory] got in there he was not responsible for his actions." The defense will produce one psychiatrist and one psychologist to support its case. The prosecution plans to have three psychiatrists testify in its behalf.

"I guess it will really be a battle of the psychiatrists," said Thomas D. Horne, the Loudoun County commonwealth's attorney. In his opening statement Horne said the shooting was premeditated.

"The defendant's actions were not those of an insane person. They may be the action of a jealous person," said Horne. "He stands before you as a person who committed cold-blooded murder."

Gregory is not being tried for murder, but for three other felonies connected with the incident: attempting to commit first-degree murder, breaking and entering with intent to commit murder while armed with a deadly weapon and use of a firearm in the commission of attempted murder.

Horne said he expects to try Gregory later on a charge of murdering LaBove. The murder charge has been dropped in the current trial because the law says spouses may not testify against each other. Mrs. Gregory is the only eyewitness to the slaying of LaBove.

During the present trial, which is expected to last at least four days in the white-pillared Leesburg Court House, Gregory's defense attorneys are expected to try to question Mrs. Gregory about any relationships with other men besides LaBove. Lawyers in Leesburg who dropped in during the first day's proceedings speculated that the defense will be depending on some of the jurors to weigh the facts on an old-fashioned scale of traditional morality.

"Picking the jury may be the most important aspect of this case," said one Leesburg lawyer who stopped by to hear one of the trial's first witnesses, Patricia Powell, who worked with Gregory at the Middleburg Training Track.

The morning after the shooting, Powell said, she saw Gregory near one of the 11 barns at the center. "I said, 'Hey Ted, I heard you shot somebody last night,'" Powell testified. "He said, 'Why don't you mind your own business?"'