In opening statements on Monday, Aaron McKinney's lawyer argued that McKinney snapped after a sexual advance from Shepard triggered memories of a homosexual assault by a childhood bully. The lawyer also contended McKinney was under the influence of drugs and alcohol at the time.

The defense is hoping to save McKinney's life by convincing the jury that he is guilty of manslaughter instead of murder.

Legal analysts said the defense has an uphill battle because of key evidence that indicates premeditation, such as taking Shepard to a remote spot to rob him.

``I don't know if this will be effective. If you assert the gay panic defense, you have to have the facts to support that,'' legal analyst Andrew Cohen of Denver said Tuesday.

Brian Levin, director of the California-based Center on Hate and Extremism, said McKinney's lawyers will have to proceed carefully.

``Jurors want to see some type of contrition, some possible explanation for the crime, but they don't want to see a victim blamed,'' he said.

Last October, Shepard was lashed to a wood fence in a remote area, robbed of $20 and pistol-whipped into a coma. He died five days later.

Prosecutors said McKinney led the charge. His co-defendant, Russell Henderson, pleaded guilty to murder and kidnapping in April and is serving two life sentences. Henderson is expected to testify.

During testimony on Tuesday, Coroner Patrick Allen said the young man's skull was fractured a half-dozen times and some of the blows may have been inflicted with the butt of a .357 revolver.

Shepard also apparently tried to ward off the blows, suffering bruises on his hands, arms and body. ``These were significant, serious blows that required a significant amount of force,'' Allen said.

``Homosexual panic'' is built on the theory that a person with latent homosexual tendencies will have an uncontrollable, violent reaction when propositioned by a homosexual.

Ralph Slovenko, a professor of law and psychiatry at Michigan's Wayne State University, said challenging a person's sexuality can be more disruptive than religious or racial insults.

Levin said the strategy has been meeting with less success in recent years because Americans have become more tolerant of homosexuals.

``I feel we've turned a very big corner in that nearly everyone agrees that violence against them is completely wrong,'' he said. ``I don't think you had the same type of atmosphere 20 or 30 years ago.''

The defense has been used in several trials, with limited success.

In 1992 in Toronto, a 19-year-old man was acquitted of fatally stabbing his boss. He claimed he was in fear for his life and operating out of homosexual panic.

In Alabama, Steven Eric Mullins, 24, used the defense but was convicted of capital murder in the Feb. 19 slaying of Billy Jack Gaither, who was beaten with an ax handle and his body burned atop kerosene-soaked tires.

In Pontiac, Mich., Jonathan Schmitz used the defense after he fatally shot a gay acquaintance who revealed a crush on him during a taping of ``The Jenny Jones Show.'' He was sentenced in September to 25 years to 50 years in prison.

``The Jonathan Schmitz case was a perfect example where introducing elements of homosexual panic won him a second-degree conviction instead of a first-degree conviction,'' said Jeffrey Montgomery, executive director of the Detroit-based Triangle Foundation, a gay rights organization.

Montgomery complained that the tactic amounts to blaming the homosexual victim.

Prosecutors typically counter gay panic defenses by asking juries to focus on the time that the perpetrator had before carrying out the attack.

In the McKinney case, the prosecution will emphasize how long it took McKinney and Henderson to drive Shepard from a downtown Laramie bar several miles to a trail leading to the rocky foothills east of town.

Jeremy Herrera, 19, said McKinney was arguing with Emiliano Morales III when McKinney went to his truck, pulled out a gun and hit Morales from behind.

``I heard a thud. It was the most sickening thing I ever heard in my life,'' said Herrera, who added he struck McKinney with a thick stick.

Police said the fight helped them connect McKinney to Shepard's beating.

McKinney is charged with murder, kidnapping and robbery. If convicted, he faces the death penalty.

During the first full day of testimony, jurors also heard from the coroner who examined Shepard's body, a bartender and Morales, who said the beating rendered him unable to identify his attacker.

McKinney's co-defendant, Russell Henderson, pleaded guilty to kidnapping and felony murder in April and received life in prison.

Prosecutors say McKinney and Henderson kidnapped Shepard and took him to a remote area near Laramie on Oct. 7, 1998 and lashed him to a fence, where they robbed him of $20. Shepard was beaten into a coma. He died five days later.

Aaron McKinney's lawyers have employed a risky strategy often called the ``gay panic'' defense to convince a jury that he is guilty of manslaughter rather than murder.

The strategy is built on the theory that a person with latent homosexual tendencies will have an uncontrollable, violent reaction when propositioned by a homosexual.

McKinney's lawyer have argued their client snapped during a drunken, drug-induced rage after a sexual advance from Shepard triggered memories of a childhood homosexual assault.

Experts warned Tuesday that employing the ``gay panic'' defense is perilous since it can amount to blaming the victim for provoking his own assault.

Brian Levin, director of the California-based Center on Hate and Extremism, also said the strategy hasn't worked recently because Americans have become more tolerant of homosexuals.

``I feel we've turned a very big corner in that nearly everyone agrees that violence against them is completely wrong,'' he said. ``I don't think you had the same type of atmosphere 20 or 30 years ago.''

That tolerance was amplified in part by the brutality of Shepard's death. Last October, the college freshman was lashed to a wood fence in a remote area, robbed and pistol-whipped into a coma. He died five days later.

Prosecutors said McKinney led the charge. His co-defendant, Russell Henderson, pleaded guilty to murder and kidnapping in April and is serving two life sentences.

During testimony at McKinney's trial on Tuesday, two policemen told how they found a .357 revolver ``covered in blood'' in a pickup used by the two defendants. Prosecutors say the gun was used to beat the 5-foot-2, 105-pound Shepard.

Larimer County, Colo., coroner Paul Allen testified that Shepard was pummeled so badly his skull was fractured in at least four places.

Prosecutors facing a ``gay panic'' defenses often ask juries to focus on the time that the perpetrator had before carrying out the attack. In the McKinney case, prosecutors plan to emphasize that he and Henderson drove several miles to the rural area where Shepard was beaten.

That would indicate some premeditation and imperil the defense's theory, legal analyst Andrew Cohen said Tuesday. ``I don't know if this will be effective. If you assert the gay panic defense, you have to have the facts to support that,'' he said.

The ``gay panic'' defense has been used in several recent trials, with limited success.

In 1992 in Toronto, a 19-year-old man was acquitted of fatally stabbing his boss. He claimed he was in fear for his life and operating out of homosexual panic.

In Alabama, Steven Mullins, 24, used the defense but was convicted of capital murder in the Feb. 19 slaying of Billy Jack Gaither, who was beaten with an ax handle and his body burned atop kerosene-soaked tires.

In Pontiac, Mich., Jonathan Schmitz used the defense after he fatally shot a gay acquaintance who revealed a crush on him during a taping of ``The Jenny Jones Show.'' He was sentenced in September to 25 years to 50 years in prison.

``The Jonathan Schmitz case was a perfect example where introducing elements of homosexual panic won him a second-degree conviction instead of a first-degree conviction,'' said Jeffrey Montgomery, executive director of the Detroit-based Triangle Foundation, a gay rights organization.