Negotiators for President Reagan and Democratic presidential nominee Walter F. Mondale agreed yesterday to at least one debate between the two candidates this fall but remained far apart on other issues after an 80-minute opening session.

"We are not only willing but pleased to agree to at least one debate, and we have not foreclosed the possibility of more than one debate," White House chief of staff James A. Baker III told reporters afterward.

But Mondale campaign chairman James A. Johnson said the two sides are "quite far apart on some of the basic questions," including the number, format and timing of presidential and vice-presidential debates.

Mondale has challenged Reagan to six debates, each on a different subject, and Johnson repeated the challenge yesterday. "But we made it clear we were interested in substantial presidential debating and we didn't want to get preoccupied with the numbers," Johnson said.

He added that Mondale's position is that if there are fewer than six debates, he would like them to be meatier and longer.

Reagan has said six debates would bore the public. White House officials have said their goal is two presidential debates and one vice-presidential debate.

Baker said after yesterday's meeting that "the question of a vice-presidential debate is still being debated. There is a possibility of a vice-presidential debate. The vice president himself will have to decide whether or not to debate."

When Mondale chose Rep. Geraldine A. Ferraro (D-N.Y.) as his running mate, Vice President Bush at first appeared reluctant to debate, but aides said later that he intended to debate her.

Johnson said Mondale would like two vice-presidential debates. He said the Reagan team "refused to say whether they would let Bush debate." Later, White House officials said Baker wants to resolve details of a presidential debate before discussing a vice-presidential debate.

Baker said the Reagan team wants a debate "not too close to Election Day so there is no undue impact on the voters' decision." Other Reagan campaign sources said the goal is for the debates to be finished a month before Nov. 6.

Mondale, by contrast, wants to space out the debates during the fall campaign. Johnson said Mondale considers a debate at the end of October "appropriate."

Reagan capitalized on such a late debate with President Carter in 1980. Baker was his chief negotiator for that debate as well.

Both sides also appeared to differ on the format of the debates. Johnson said the Reagan camp wants "something very close to 1976 or 1980. I indicated Mondale would prefer more flexibility, more give and take and a format that covers more issues more quickly."

Another Mondale adviser said, "I think we want a Dan Rather type of debate," referring to the New York primary session Mondale had with Sen. Gary Hart (D-Colo.) that was marked by the sharpest exchanges of any of the primary debates. It was moderated by CBS News anchorman Dan Rather.

The meeting yesterday was held at the law offices of Dean Burch, a longtime informal Bush adviser, and campaign advisers spoke to reporters on a street corner afterward.

Reagan was represented by Baker, Burch and Frank Donatelli, a White House assistant who was substituting for Richard G. Darman, who also will be part of the Reagan team. Mondale was represented by Johnson, former Carter White House aide Anne Wexler, now a senior adviser to Ferraro, Richard Leone and former senator John C. Culver (D-Iowa).

The two sides agreed to meet again in the next few days.