Theodore C. Gregory, a Loudoun County horse trainer, was convicted today of voluntary manslaughter for shooting to death his wife's lover when he found the two in bed together. Gregory left the courthouse a free man after promising to pay a $1,000 fine for the offense.

A six-man, six-woman Rappahannock County jury deliberated for four hours before rejecting a first-degree murder charge against Gregory that could have sent him to prison for life.

The 31-year-old Gregory, who was jailed for 22 months without bond in the hunt country shooting, smiled broadly as the verdict was read, and members of his family in the courtroom began to cry quietly. Before leaving the courthouse, Gregory said he had no comment other than: "I think the way the jury put it was exactly right."

Voluntary manslaughter, punishable by up to 10 years in prison, was the least serious offense of which the jury could have convicted Gregory, and a $1,000 fine is the minimum penalty for it under law. The defense asked for acquittal, arguing that the slaying was the result of an "irresistable impulse" that struck Gregory when he found his estranged wife, Monique Dana, making love to Howard LaBove, who was described at the trial as Gregory's best friend.

LaBove, a fellow horse trainer in the moneyed, elite world of racing and steeplechase riding, died after being shot three times with Gregory's .45-caliber pistol.

Under Virginia law, voluntary manslaughter is intentional killing in "the sudden heat of passion upon reasonable provocation."

Jury foreman Paul Poling said after the verdict that "it was the question of the degree of murder that really pulled" the jury to its decision. The jurors decided, he said, that the elements of manslaughter as defined by the judge most accurately fit the event.

The jury's recommended $1,000 fine was promptly imposed by Judge Carleton Penn, who gave Gregory 15 days to produce the money.

The slaying occurred on the night of Aug. 20, 1980, when Gregory barged in on Dana and LaBove at LaBove's rented cottage near Middleburg.

Dana, 28, who testified for an hour on Wednesday as the prosecution's star witness, was not in the courtroom when the verdict was announced.

Friends of both Gregory and LaBove testified they grew increasingly concerned about Gregory's state of mind after his separation from Dana in May 1980 and had warned LaBove about dating her. Asked if he was shocked when he heard about LaBove's death, horse dealer Joseph Fiore said in court, "No, because I told everybody it was going to happen."

Judge Penn had ordered the trial moved to this tiny Blue Ridge Mountain town 80 miles west of the District of Columbia to escape the impact of publicity about the case in Loudoun County.

Gregory was convicted in late 1980 of trying to murder Dana, but won acquittal in a retrial in May. Since Dana by that time was divorced from Gregory and thus free to testify against him for the prosecution, Loudoun prosecutors sought and obtained the murder indictment for LaBove's death.

Gregory's older brother, Joseph, said in an interview this week that his family spent a total of more than $50,000 on his brother's defense over two years, including a $3,000 fee for a one-hour appearance on Thursday by a psychiatric expert. The money was raised, he said, by mortgaging the family farm and through income from his father's Fauquier County construction business.

In final arguments today, assistant Loudoun prosecutor James E. Mechling accused defense lawyers of trying to shift the focus of the trial away from the fatal shooting to Dana's affair with LaBove and her alleged relationships with other men while still married.

"That is a smokescreen, a red herring," Mechling declared. "Gregory carried out his plan to its ultimate, final, bloody, tragic conclusion." Facing the jury, Mechling added, "Murder. Murder. Murder. It's an ugly word. It's an ugly deed."

"The evidence screams for doubt," responded chief defense lawyer Blair Howard. He pictured Dana as someone who "chose life in the fast lane," leaving her then-husband Gregory "devastated" and "pathetic."

Quoting Shakespeare, Howard asked the jurors to view Gregory as "one who loved not wisely, but too well. . . not easily jealous, but wrought, perplexed, in the extreme." Gregory's brother and father wept and the normally reserved Gregory wiped his eyes.

In day-long psychiatric testimony on Thursday, experts for the two sides agreed that Gregory had been depressed, anxious and grief-stricken for weeks leading up to the shooting, but disagreed sharply in their conclusions about Gregory's state of mind on the fatal night.

Defense psychiatrist David Lanham said he found Gregory to be suffering from clinical depression and a long-term schizoid personality disorder. The doctor described Gregory as reserved, aloof and unable to express his feelings -- a man, Lanham said, who had told his wife only once in six years of marriage that he loved her, "although his love was very deep."

Prosecution medical experts testified that Gregory's behavior failed to rise to the level of mental disease or disorder. The shooting "was an act unresisted and not irresistable," said Dr. James Dimitris of Virginia's Central State Hospital.