A Gaithersburg woman who had filed an application with a computer dating service asked the company's representative to kill her ex-husband for $5,000, a prosecutor told a Montgomery County Circuit Court jury yesterday.

The woman, Ruth Betty Blosser, 51, has pleaded innocent to charges of solicitation to murder and maim. Her attorney told jurors as her trial began yesterday that she was entrapped by police and the dating service representative, a man with a long criminal history who was then on parole for bank larceny.

According to prosecutor John McCarthy, a representative on his second day of work for a Laurel dating service came to Blosser's apartment on Club House Road in Gaithersburg last March with forms for her to fill out.

According to McCarthy, Blosser, a secretary, spoke bitterly of her ex-husband and mentioned that he had once had her arrested for trespassing. The representative, Vincent Michael McCarthy (no relation to the prosecutor) said that he, too, had been in jail, and that Blosser then asked him to kill her husband, the prosecutor said.

The two then went to the bedroom and engaged in sex, according to John McCarthy and later testimony by Vincent McCarthy, the representative, who said he then drove to the Montgomery County police station in Bethesda and reported what happened.

Police asked Vincent McCarthy to keep in touch with Blosser by telephone, the prosecutor said. In August, the representative was wired with a microphone and went into the woman's apartment, where, prosecutor McCarthy said, the man was given three rings as a down payment for the killing. Police were listening to the conversation from the street, the prosecutor said, and after the man left, they entered the apartment and arrested Blosser.

Defense attorney Joseph DePaul said Blosser had every reason to hate her husband, a hospital administrator in Ohio. Blosser's husband beat her, beat his children and cheated her out of money, DePaul said.

But he said Blosser was entrapped by a man who was anxious to go along with police in an effort to get released from parole.

"He's a pretty smooth cookie," DePaul said of the dating service representative. "He conned a 20-year probation officer into cutting him loose. He got a lot out of this."

Although Vincent McCarthy was released from parole soon after Blosser's arrest, his probation agent testified yesterday that the release date was "standard" and had been discussed with McCarthy when he was released from jail in 1981.

DePaul called Blosser "a decent, middle-aged woman in a bad situation" who was a particularly susceptible subject for entrapment. He said psychiatric evidence to be introduced later in the trial will show that she is a "passive, dependent type who is very easily led astray, and very easily dominated."

But John McCarthy told jurors that the case revolves around "cold, calculated, premeditated contract murder -- murder for hire."

The jury is expected to hear today the taped conversation between Blosser and the representative. John McCarthy told jurors that enough of the tape is audible to indicate Blosser's serious intentions.