Monique Dana, clutching the witness chair and shaking uncontrollably, defended her role in a fatal love triangle today in the first-degree murder trial of her ex-husband, Middleburg horse trainer Theodore Gregory.

Her heels tapping the wooden courtroom floor in nervousness, Dana, 28, told the jury Gregory flung open the door and opened fire in the bedroom where she and her lover, Howard LaBove, were having intercourse. LaBove was killed in the melee.

Gregory, 31, who has pleaded temporary insanity in the killing and who faces 20 years to life in prison if convicted, stared impassively at his former wife as she described the events of the fatal night in August 1980.

Testimony by Dana, who repeatedly snapped at Gregory's lawyers, highlighted the trial's second day, during which a parade of witnesses from Middleburg's hunt country world of trainers and jockeys portrayed Gregory as a man shocked and obsessed by his separation from his wife earlier that spring. "Ted became a recluse," said horse owner Joseph Fiore of nearby Atoka.

Fiore said he was startled to discover two loaded pistols, a camouflage jacket and powerful binoculars in Gregory's pickup truck several weeks before the shooting. "I looked at Polka Dot Farm about a mile away (through the binoculars) and I could see into the bedroom window," Fiore said.

Fiore's wife, Susan, testified that Gregory, who temporarily lived with the Fiores after his separation, had once donned the camouflage jacket, a camouflage cap and dark glasses and driven off in a borrowed car to "spy on his wife."

Loudoun County prosecutors presented testimony that Gregory had once threatened to kill two people, apparently referring to Dana and her lover.

C. Langhorne Washburn, former assistant Commerce secretary and owner of Holly Hill Farm near Middleburg where the Gregorys lived before their separation, said Gregory told him "he was going to shoot them both. I told him if he did that he would ruin his life, and he said, 'I'll take them both with me. I can't wait to get a divorce,' " Washburn said.

Washburn, who testified over strenuous defense objections, said the remarks followed a nighttime visit by Gregory to his estranged wife. "I think I arrived on the scene" shortly after the couple quarreled, said Washburn, who was awakened by a 2:15 a.m. telephone call and asked to escort Gregory off the property.

"She [Dana] was bleeding from the mouth where he struck her," Washburn added before defense lawyer Blair Howard leaped to his feet to object. Circuit Judge Carleton Penn told the jury to disregard the remark.

Penn ordered the trial moved to this Rappahannock County town, which touts itself as "The First Washington of All," because of publicity surrounding Gregory's earlier trials. Gregory was convicted two years ago of attempting to murder Dana but was acquitted in a retrial.

Gregory, under court order to stay away from his former wife, looked on today as she argued frequently with defense attorney Howard over details of the shooting and her relationships with other men.

"I wonder if I'm on trial here," she lashed out at one point. "I thought we were trying to find out if he killed somebody."

Howard focused repeatedly on Dana's ties to a Middleburg exercise rider during May 1980, the same month the Gregorys separated. Dana described the man, Dale Smallwood, as "like a brother" and denied Howard's suggestion she had attempted to hide the relationship from the public.

Smallwood's employer, Daniel Yovanovich, testified, however, that Dana frequently telephoned Smallwood a half-dozen times a day. "It interfered with Dale's work," said Yovanovich, who added that Smallwood was "crazy about her."

In late May, Yovanovich said, he and his wife, together with Smallwood and Dana, left Middleburg to drive to annual steeplechase races at Fair Hill, Md. As they traveled through Middleburg, he said, Dana asked Smallwood to lie down on the car seat so the two would not be seen together.

Several defense witnesses testified Gregory confronted Dana at Fair Hill and later broke down in tears after finding contraceptives in her purse.