IN HIS LITTLE prison cell, the fevered brain of Leonard Jenoff whirs and spins, and who knows what fantasies spill out among the occasional truths? Rabbi Fred J. Neulander might know, but he decided to keep quiet. This leaves it up to a New Jersey jury to mull over the killing of the rabbi's wife, and the connection to Jenoff, the former Baltimore police operative now turned confessed killer.

The case has been front-page stuff now for several years - especially since Jenoff confessed two years ago that he walked into Rabbi Neulander's house and bludgeoned to death poor Carol Neulander, the rabbi's wife.

Jenoff, the former Baltimorean and well-documented liar, says he killed Carol Neulander as a favor to the rabbi, who allegedly described his wife as an "enemy of Israel." The rabbi says this is fantasy. Jenoff says the rabbi promised him $30,000 for the job. The rabbi denies this, too.

On the witness stand a week ago - the trial's been running on cable's Court TV for weeks now - Jenoff made a pretty convincing case. He said the rabbi paid him big money to do it, and he offered the jury some colorful details. He said the rabbi needed his wife of 29 years eliminated so he could carry on an affair with a Philadelphia radio host named Elaine Soncini.

It is all dramatic and grisly business, but who knows how much is true? That the rabbi's wife has been killed, there is no doubt. And, by Jenoff's own confession, he and another fellow, a roommate named Paul Michael Daniels, committed the deed.

But the rabbi denies that he urged them to do it or that he financed it. And, for all the convincing details furnished by Jenoff, there is that little business of his personal history, with which the jury now has to grapple. Jenoff is a liar, by other people's accounts and, on the witness stand, by his own account.

"A delusional serial liar [who] ... doesn't know where the truth ends and fantasy begins," Michael Riley declared during closing arguments last week. Riley is Rabbi Neulander's attorney. The rabbi's best defense is Jenoff's history of lies, his desperation for attention.

Among those covering the trial is Baltimore author Arthur Magida, who's writing a book about the case. On the telephone Friday, Magida related a story. During a courtroom break, he leaned over to talk to Jenoff's lawyer, Frank Hartman.

"Oh, good," Hartman said, "[Jenoff] wanted to know if you were here." Jenoff knew Magida was writing a book, and it pleased him. It was proof he'd finally done something of lasting importance.

"And Dateline," Hartman told Magida. He meant the NBC-TV show. "He wants to know if Dateline's here." Jenoff has agreed to be interviewed by the TV program. In the midst of testifying at a murder trial, a television appearance is his big concern.

Who knows if they'll get the truth out of him? Jenoff's lying goes back at least as far as his Baltimore days, when he worked as a free-lance, paid undercover operative for the city Police Department. Those were the days when then-Commissioner Donald D. Pomerleau had his intelligence unit breaking all kinds of rules. The intelligence unit was called the Inspectional Services Division. They reported directly, and only, to Pomerleau. Insiders said its initials, ISD, meant "I Serve Donald."

Jenoff was one of its out-of-control minions, a James Bond wannabe who didn't care what laws he broke. It was part of the fun. Sometimes, he'd sit around with law enforcement types and try to be a real insider. He'd brag about his alleged background. He'd talk about his history with the CIA. This was based strictly on fantasy. He'd talk about his Baltimore police background. This was based on fact - but, how much was fact, and how much was fantasy, nobody was entirely sure.

Some of this history came out on the witness stand in recent days. There was Jenoff, looking old and bulky and utterly pathetic in his jailhouse outfit, a man ruined by his inability to tell the truth, and his neediness, and his willingness to commit murder.

Two years ago, Jenoff admitted to a Philadelphia Inquirer reporter, Nancy Phillips, that he'd committed the murder of Mrs. Neulander. He said he needed to confess to clear his conscience. The next bombshell was his accusation that the rabbi had put him up to it.

But a first trial ended in a hung jury - reportedly, a 9-to-3 vote to convict the rabbi. At the retrial, there was one big difference. The rabbi chose not to testify in his own behalf. He sat there every day, mostly expressionless. So he and Jenoff share a secret: Only they know if the rabbi really paid him to kill Carol Neulander.

But only Jenoff truly knows the strange whirs and spins of his own little brain, which produces so much fantasy among the occasional truths.