Ambassador Richard Stone, President Reagan's special envoy to Central America, today held his first substantive talks with representatives of the Salvadoran guerrilla movement, but the meeting apparently ended inconclusively with only promises that "communications" between the two sides would continue.

Costa Rican President Luis Alberto Monge, who helped arrange the meeting, read journalists a terse, three-paragraph joint communique at the end of the three hours of discussion. It said the talks had been held with an "open agenda" and in an atmosphere of "frankness and mutual respect." He read the communique in his presidential palace with both delegations sitting across a table from each other.

"In it," the communique stated in its only indication of the contents of the discussions, "it was agreed to continue communications."

Neither U.S. nor guerrilla representatives would explain why the communique had used the word "communications" instead of "dialogue," which would have indicated that further meetings were planned.

U.S. officials declined to make any declaration beyond the communique language about the talks that were held in a private home on the outskirts of this Central American capital.

But members of the Salvadoran rebel negotiating team, headed by Guillermo Ungo, head of the guerrillas' political arm, the Democratic Revolutionary Front, expressed optimism that the talks they had long asked for had finally taken place even though there was no indication that any progress had been made.

Both Salvadoran and U.S. sources here indicated privately that all that had transpired is that both teams had simply restated their still widely divergent positions on how to solve the Salvadoran conflict in a face-to-face encounter.

The Salvadoran opposition representatives later told journalists that they considered that the meeting "broke the ice" for future negotiations. They termed the talks today "a first step" of what they hoped would be many more such meetings.

But Ungo, who led the opposition's four-man delegation, declined to comment on whether further meetings had been agreed to and said "no date for other meetings" had been set.

In a separate news conference after Monge had read the joint communique, Ungo made it clear that although there had been an agreement not to discuss "the content" of the talks, the Salvadoran rebels had not in any way changed their opposition to participation in elections next year in El Salvador under the present government.

Stone and the U.S. administration had maintained all along that the only object of their talks with the combined representatives of the Democratic Revolutionary Front and the guerrillas' military Farabundo Marti National Liberation Front was to coax them to agree to participate in the elections.

"We insisted that the elections convoked for next year are not a solution for El Salvador , but a false alternative," Ungo said at the second news conference that was attended by two other members of his delegation--Ruben Zamora, a Christian democrat, and Mario Aguinada of the Communist Party of El Salvador, a guerrilla group. Another guerrilla representative, Mario Lopez, also attended the talks with Stone.

Ungo and Zamora both reiterated that their front insisted there could be no elections in El Salvador until a "new, broad-based and pluralistic" transition government could be set up in which they would be represented.

Zamora, who had originally met with Stone in Bogota, Colombia, July 31 to set up today's talks, insisted today that the two opposition coalitions considered elections "to be a necessary part of any political solution." But he said elections could only be held under a new government that could realistically guarantee their openness as well as the safety of all candidates and party workers of the left who might take part in them.

"I can tell you flatly we are not going to participate in the elections as planned for next year," Zamora said. "Because if we did under the present government and conditions, hundreds of us would be killed by death squads."

Zamora termed the United States "the key to a solution" and said that so long as the Reagan administration doesn't honestly accept "the need for a negotiated political solution" the conflict in El Salvador could not end peaceably.

Because of that, he said, he considered direct talks with the United States a "parallel but separate" process for finding a solution, distinct from the talks between the Salvadoran opposition and a government peace commission yesterday in Bogota, Colombia.

Zamora, however, pointed out that the peace commission was not considered a "true interlocutor" for the Salvadoran government because it had no power beyond the advisory.