NEW BRUNSWICK, N.J. — No one disputes that Melanie McGuire bought a .38-caliber revolver a few days before her husband was shot to death. What is in question at her murder trial is whether she purchased that weapon to kill her spouse or to do him a favor.

McGuire's lawyers maintain her husband, William, asked her to buy the gun for family protection because, as a convicted felon, he was prohibited from owning a weapon.

On Monday, lawyers for McGuire opened their defense and, perhaps underscoring the importance of the gun evidence, called as one of their first witnesses a man who said he and the victim discussed the purchase of a firearm a few months before his death.

The witness, George Lowery, an information technology specialist, worked with William McGuire, a computer analyst for the New Jersey Institute of Technology, on a state program involving the computerization of health records. He said McGuire visited him in his office at the Camden County health department once every one or two months and that, during their meetings, they occasionally discussed personal matters.

In early 2004, about three months before William McGuire went missing, the talk turned to buying a gun, he said.

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State Superior Court Judge Frederick DeVesa permitted Lowery to testify only to the fact of the conversation, not its contents. In a hearing outside the presence of jurors, the defense said Lowery was prepared to say that William McGuire told him that he needed a gun because his car headlights had been stolen several times and that he planned to ask his wife to buy it for him because he could not get a permit.

DeVesa, however, agreed with prosecutors that relating the victim's statements would be hearsay.

"Mr. McGuire's state of mind prior to the murder is really not that significant," DeVesa told the defense.

Melanie McGuire bought a gun and ammunition at a Pennsylvania gun shop on April 26, 2004. Her husband was last seen alive two days later. His remains were found in the Chesapeake Bay the next month.

In e-mails to a friend before her husband went missing, Melanie McGuire said she needed a gun to protect herself from her husband's increasingly strange behavior, but after her husband's death, she told the same friend that her husband had asked her to get the gun because the family was moving to a more rural area and he was worried about safety. William McGuire was prohibited from owning a gun after he was convicted of perjury in 1997.

The defense called as its first witness a speech therapist who tutored the McGuires' two young sons. Jo-Anne Cascia testified that McGuire phoned her April 29, 2004, to cancel one son's appointment. Cascia said McGuire explained that she and her husband had a fight.

"He had walked out on her and she had not yet heard from him at the time I talked to her," Cascia recalled.