The chairman and vice chairman of a special Senate subcommittee assigned to investigate Billy Carter agreed yesterday that President Carter will have to be questioned, and both said they would prefer that he testify in person.

Subcommittee chairman Birch Bayh (D-Ind.) indicated that he also thought Rosalynn Carter and national security adviser Zbigniew Brzezinski should be called as witnesses.

"I frankly think that looking a guy in the eye or a woman in the eye is the best way" of taking a testimony, Bayh told reporters. He added, however, that the nine-member subcommittee would have to settle the question.

Standing alongside Bayh following an uneasy meeting of the Senate Judiciary Committee, the subcommittee's vice chairman, Sen. Strom Thurmond (R-S.C.) voiced his agreement, at least as far at the president is concerned.

"If it's apparent that the president is in any way involved, I think it would be to his advantage to appear in person," Thurmond said.

The Judiciary Committee meeting, which produced the appointment of seven of the subcommittee's members, was laced with expressions of discomfiture by Democratic senators in light of the timing of the inquiry. The Democratic National Convention in New York City begins in 16 days.

"We are not here to repay Watergate, gentlemen," Bayh protested after a partisan speech by Sen. Bob Dole (R-Kan.) comparing those scandals with what he called "Billygate."

Bayh suggested later that the major difference between the Watergate investigations and this inquiry is that "this one takes place just before an election."

As a consequence, Bayh said he and Thurmond were "dedicated to disciplining ourselves" and producing an impartial inquiry.

Dole, who was also assigned to the subcommittee, showed no such reserve. He said Watergate pitted a Republican White House and Justice Department against a Democratic Congress while "in Billygate, as it's been called, Democrats control the White House, Congress and the Department of Justice."

While Watergate involved dirty campaign tricks and efforts to cover them up, Dole continued, "so-called Billygate involved relatives of the president, White House officials at the highest level, personal financial gain including the possibility of indirect financial gain for the president himself, and apparent favoritism by the Justice Department in allowing Billy Carter to enter into a consent decree. There are important foreign policy considerations in Billygate. There were none in Waterrgate."

The stinging summary clearly nettled Bayh and other Democrats, who emphasized that the Carter White House, unlike the Nixon White House, has promised its cooperation and does not expect to assert claims of executive privilege.

"I don't know what the parallels [with Watergate] are," Bayh said. "I hope we don't find high officials culpable."

The Senate voted Thursday to conduct the investigation of Billy Carter's relationship with the government of Libya after the president's brother was forced to file as a foreign agent for that nation last week. He disclosed that he had been paid $220,000 by the Libyans this year and subsequently said he used the money to pay off various debts and obligations.

In speaking of "the possibility of indirect financial gain for the president," Dole was evidently alluding to speculation that some of the money may have been used to pay off debts to the Carter family's peanut warehouse, in which the president is a major partner. White Hose press secretary Jody Powell, however, has quoted the trustee of the president's interests. Atlanta lawyer Charles Kirbo, as saying that none of the Libyan money has been used to reduce Billy's indebtedness to the warehouse.

Yesterday's Judiciary Committee meeting to announce the membership of the special subcommittee had been set for 9:30 a.m., but it was delayed until midafternoon because of difficulties in finding enough senators willing to serve. Bayh had hoped to persuade Sen. John Culver (D-Iowa) to fill one of the Judiciary Committee's seven slots on the panel, but Culver, who is facing a stiff fight for reelection, turned him down.

Two other members of the subcommittee were to be chosen from the ranks of the Senate Foreign Relations Committee by Senate Majority Leader Robert C. Byrd (D-West Va.) and Senate Minority Leader Howard H. Baker Jr. (R.-Tenn.), but they have yet to name their picks.

Baker was apparently trying to get Sen. Jacob K. Javits (R-N.Y.) but Javits, who is also running for reelection, told a reporter he was still undecided.

A member of the Senate Watergate committee, Baker recalled that there was similar reluctance to serve on that panel. "They used to call it 'scott's revenge,'" he said of the Republican appointments that were made by then -- Senate minority leader Hugh Scott.

The Judiciary Committee appointees announced by Bayh and Thurmond were Sens. Patrick J. Leahy (D-Vt.), Max Baucus (D-Mont.) Dennis DeConcini (D-Ariz.) Charles McC. Mathias Jr. (R-Md.) and Dole.

"We have a difficult job to do," Bayh, who is also running for reelection, said at the outset of the meeting.

"Nobody likes it. But somebody has to do it . . . It's not easy in an election year. It tests us all the more."

Some Democrats might perceive themselves as benefitting from the television exposure and the distance that the inquiry might provide between them and the president. On the other hand, Baucus told reporters he had agreed to serve only reluctantly because "it's a no-win situation. And it's going to take an awful lot of work."

The full subcommittee is expected to meet Tuesday to consider a preliminary staff report proposed by Thurmond. It will include "a summary of the facts known to date, the issues raised by these facts, the areas that need further factual investigation," and a legal analysis of applicable statutes. Staff lawyers were authorized to begin preliminary interviews with White House, Justice Department and other administration officials.

The meeting took place before members were aware of Attorney General Benjamin R. Civilett's belated disclosure that he told the president on June 17 that Billy Carter probably would not be prosecuted if he would register as a foreign agent.

"It's this kind of thing that points out the need for this special subcommittee," Bayh said last night of Civilett's revelation. "And it's this kind of thing that we'll be looking into."

The Indiana Democrat said, however, that he did not think it could be possible to hold more than one or two brief hearings before the convention. At that point, he acknowledged, a lot of the issues raised by Billy Carter's activities will remain up in the air, but he added: "A lot of them are up in the air now, aren't they?"

Asked where the hearings would be held, Bayh quipped: "We thought perhaps RFK Stadium."

Mathias stressed the need for a determined, yet evenhanded approach.

"Whatever we do can have an immediate effect on the history of this country -- because it may have an effect on the election forthcoming," he said. "If we're overly aggressive, it may create a climate of sympathy for those under investigation that they do not deserve. If we are not aggressive enough, it may give rise to charges of cover-up."