A federal judge here questioned 34 prospective jurors behind closed doors yesterday, but no one was picked to sit on the jury hearing the case against three Cuban exiles charged in connection with the 1976 bombing assassination here of former Chilean ambassador Orlando Letelier.

U.S. District Court Judge Barrington D. Parker excused six of the propsective jurors after they described to him what they had read or heard about the case. Parker earlier excused one man as a potential juror after he said he personally knew Letelier, an outspoken opponent of the Chilean military dictatorship of Gen. Augusto Pinochet at the time of his Embassy Row slaying.

Parker plans to continue questioning another 55 people on the jury panel today, all of whom said they had read or heard about the killing of Letelier and Ronni K. Moffitt, a colleague of his at the Institute for Policy Studies.

The arduous and lengthy task of picking an impartial jury to try the three Cubans -- Guillermo Novo Sampol, his brother, Ignacio Novo Sampol, and Alvin Ross Diaz -- forced postponement of the scheduled start today of a drug conspiracy trial.

The judge in the drug case, Louis F. Oberdorfer, had asked court officials for a panel of 120 jurors from which to pick a jury. But court clerk James F. Davey said that because of the large number of prospective jurors in the Letelier case, he could guarantee a pool of only 60 jurors for the drug case. So the drug case was postponed until Jan. 22.

Some of the jurors questioned yesterday reportedly had heard of the death threats against Parker and one of the prosecutors in the case, Assistant U.S. Attorney Eugene M. Propper. Others said they recalled the fact that the Chilean secret police, once known as DINA, is alleged to have masterminded the plot to kill Letelier.

Few reportedly had any recollection of defense allegations that the Central Intelligence Agency had a role in the killing. The government's key witness, an American-born DINA agent, Michael V. Townley, who has confessed to placing the bomb in Letelier's car, offered his services to the CIA in 1970. In February 1971, the agency's directorate of operations asked for "preliminary security approval" to use Townley "in an operational capacity," according to a CIA affidavit in the case.

But the CIA said its files do not reflect whether Townley was used as an operative and that its interest in him was canceled on Dec. 21, 1971, nearly five years before the Letelier killing.