Billy Ray Shepherd, charged with his brother, a U.S. Park policeman, in connection with the aborted plot to kidnap restaurant magnate J. Willard Marriott Sr. or his wife, pleaded guilty yesterday to conspiracy to illegal use of a telephone in the incident.

Shepherd, 52, then testified in U.S. District Court in ALexandria that his brother, Paul Dwain Shepherd, 44, lured him into the kidnaping plot. Paul Shepherd later testified that he was enticed into the plot by an FBI informant and an undercover FBI agent posing as a thug. Paul Shepherd said he feared the agent because he thought he was a Mafia hit man.

Billy Shepherd testified that his brother brought up the subject of the kidnaping in mid-July.

"He said he was involved in a kidnaping or plan to kidnap Mr. Marriott and wanted to know if I was interested," Billy Shepherd testified. "In general, he went over the plan. He was to kidnap either Mr. or Mrs. Marriott from the house and transport them in some sort of van and haul them about three blocks."

After hearing the plan, Billy Shepherd testified that he "told his brother" that sounded like a pretty serious thing to get involved in. I was a little reluctant to get involved in that. It was very reluctant."

But Billy Shepherd said he later agreed to go along with it but refused to commit the actual kidnaping.

"I did indicate to him that I would not go into the house," Billy Shepherd testified. "I would not want to go into anyone's house, especially people as wealthy as Mr. Marriott. I wouldn't know if they had any security system or machine guns or anything like that. I would just be scared to go into the house."

Billy Shepherd pleaded guilty to conspiracy to transmit telephone conversations in interstate commerce to get a $500,000 ransom in the case and was sentenced by Judge J. Calvitt Clarke Jr. to a suspended five-year prison term. Clarke placed him on probation for five years.

Clarke admonished Shepherd, a Calvert County school maintenance employee saying: "Hopefully what you did was out of character. Hopefully you have learned your lesson and you will actively seek to prevent other people from getting in the same trouble."

In return for his guilty plea, prosecutors dropped charges against him of conspiracy to kidnap, extortion, attempted abduction with the intent to extort and conspiracy to commit extortion.

Shephard said that on July 29 his pliceman brother called him and said he had been in touch with a Mafia person and that he gave the gangster Billy Shepherd's phone number as an accomplice in the alleged plot. The Mafia figure was supposed to call Billy Shepherd to make sure he was in on the plan, Billy Shepherd testified. "I was supposed to go along with this thing. Kind of string him along," he said.

Under cross-examination, Billy Shepherd said he never actually planned to go through with his part of of the kidnaping.

In Paul Shepherd's defense, two park policemen and three men who belong to Shepherd's church testified Shepherd was a good man and that they would never imagine he would try to kidnap anyone.

Paul Shepherd testified that he became involved in the alleged plot last July when he asked a convicted felon, who unknown to Shepherd was an FBI informant, how he could get a stolen tractors cheaply to give his younger brother for a housewarming gift.

In subsequent contacts with the informant, Shepherd said, he arranged to buy the tractor, not try to hire a professional criminal to help with the kidnaping, as the prosecution has alleged.

The informant suggested to him that "the big right thing now is the kidnaping business" and asked Shepherd to name some possible victims, Shepherd said. Shepherd said he suggested one of the Marriotts, either Redskins football coach George Allen or his wife or John A. Volpe, owner of a large construction company and a former Secretary of Transportation.

Shepherd ssid he tried to tell the informant he wasn't interested, but agreed to meet the informant's underworld friend, Eddie Rossi, who was actually an undercover FBI agent named Edward Robb.

"It was rather exciting to tell you the truth, a lot of mystery" associating with "an underworld figure who was a quote, unquote hit man," the Park policeman said.

The case is expected to go to the jury today.