Yasser Arafat, chairman of the Palestine Liberation Organization, said in an interview today that he is ready to accept explicitly a key U.N. Security Council resolution recognizing Israel's right to exist if the United States explicitly endorses the right of the Palestinian people to "self-determination."

Arafat's qualified endorsement of Security Council Resolution 242 in a post-midnight interview appeared designed to improve chances for a dialogue between the PLO and the United States by taking a step toward accepting a key American demand.

Washington has refused since 1975 to have official contacts with the Palestinian political leaders until they explicitly accept Israel's right to exist as enshrined in the U.N. document.

There was no immediate American response to the statement by Arafat, who has sought in the past to blur his own stand on Resolution 242 and who as recently as March refused to address that question directly in interviews.

The Reagan administration has rejected a specific endorsement of Palestinian self-determination since it could lead to the establishing of an independent Palestinian state on the occupied West Bank and Gaza Strip territories. But some officials in the administration had cited the kind of formula voiced by Arafat today as a key to moving the stalled Middle East peace process off dead center.

But skepticism remains pronounced on both sides. A senior American official in the region said last week, "Maybe if we had a tape recording of Arafat saying, 'I accept 242' locked in a vault in New York, then maybe -- maybe -- we would see some room to move. But there is just no trust there."

Arafat, speaking to reporters from The Washington Post and The Los Angeles Times, also voiced pessimism about any immediate breakthrough. Despite the talks here this week between Secretary of State George P. Shultz and Jordan's King Hussein, "nothing has changed" to push ahead the three-month-old joint Jordanian-PLO peace initiative, he said.

But when Arafat was questioned this morning about the key U.N. resolution, which calls for Israel to return lands occupied in the 1967 Arab-Israeli war in return for peace agreements, he responded in uncharacteristically direct fashion.

Asked if he would explicitly accept 242 by name if the United States explicitly endorsed Palestinian "self-determination," he said, "yes" and, "absolutely."

"We are not refusing 242 because it is a resolution," he said. "We are refusing 242 because it does not treat us as people."

In that case, Arafat was asked, would he accept Israel's right to exist?

"I would accept all the international legality. It is very clear what I am saying," he said. "Simple and clear. We are not against this resolution. It is simple and clear."

Arab officials insisted tonight that Arafat's statement was not in direct contradiction with the most recent public declaration on the subject by the PLO's executive committee, which on Feb. 19 rejected Resolution 242. These officials said that an American endorsement of self-determination for Palestinians, as demanded by Arafat as a condition for his endorsement of 242, would dramatically change the situation and overtake the executive committee's rejection.

Arafat was alert at the 2 a.m. interview as his aides and PLO Executive Committee member Mohammed Milhem, seated around him in a sitting room of the royal guest house, appeared to be on the verge of nodding off.

Dressed in his customary olive drab uniform and checkered khefiya headress, smiling behind his stubble of beard, he periodically spun off into rhetorical circles around the issues blocking progress toward peace among Israel, its Arab neighbors and the Palestinians it displaced.

Arafat said it took two years of talks before the Feb. 11 accord that launched the Jordanian-Palestinian initiative could be signed.

"It was not a joke, this agreement between me and King Hussein. It was very important and a very concrete agreement," he said.

For years "we were told as soon as you have this, it will facilitate everything," Arafat said. "Then when we have it, it has done nothing."

The same thing occurred with the proposals of the Arab summit conference in Fez, Morocco, in 1982, Arafat said. "We accept it, and we have done nothing."

"Now we are offering this new option," he said, "and we have done nothing. Why? Because there is something wrong in the American administration. They are completely tied to this American-Israeli connection."

As the current initiative developed, a first step toward negotiations with Israel was supposed to be talks between a joint Jordanian-Palestinian delegation and the United States. The diplomatic face-off of the last several weeks has been over which Palestinians would be acceptable as negotiators.

The PLO is officially recognized in the Arab world as the sole legitimate representative of the Palestinian people.

But the United States, by various assurances to Israel, has committed itself not to talk to the PLO directly, even as part of a joint delegation with Jordan, unless the PLO recognizes Resolution 242.

The PLO, however, has refused to accept 242 because, it maintains, while the resolution recognizes Israel's right to exist in peace, it does not recognize the Palestinians at all except as refugees essentially without a country.

The PLO wants "self-determination" for the Palestinian people. This is usually taken to mean establishment of a Palestinian state. The United States opposes this and consequently rejects "self-determination" as well.