President Reagan wants to hold a get-acquainted "meeting" with Soviet leader Mikhail Gorbachev, possibly this fall at the United Nations, but believes a full-scale "summit" conference would require more time and preparation, national security affairs adviser Robert C. McFarlane said today.

McFarlane said "there need not be a specific agenda" for a meeting, and that the climate in U.S.-Soviet relations seems to justify one, but "you shouldn't have high expectations." Soviet actions in the months ahead could affect the timing, McFarlane said.

He told reporters that the administration believes extensive talks with the Soviets would be required in advance of a summit conference, as opposed to a get-acquainted meeting. He recalled that past summits had raised expectations for improved U.S.-Soviet relations that later were dashed.

McFarlane answered questions from reporters after several days of conflicting signals from the administration about the likelihood of a meeting or summit between Reagan and Gorbachev.

Administration sources said that McFarlane's comments were intended to signal that a meeting at the United Nations this autumn is likely, but to discourage speculation that it would produce substantive results.

"There probably will be one, but don't get your hopes up for an arms control agreement," said a senior official.

Officials said that the Soviet freeze on medium-range nuclear missiles in Europe announced by Gorbachev over the weekend -- and denounced as a propaganda ploy by the United States -- would not block a fall meeting at the United Nations. But a senior official said a repeat of this episode could jeopardize any kind of meeting between the leaders.

McFarlane mentioned two possible events -- the opening of the U.N. General Assembly in September and the celebration in October of the 40th anniversary of the founding of the United Nations -- at which the two leaders could meet. The Soviets have not yet indicated whether Gorbachev will attend either.

It was the first time recently that the administration had tried to make a distinction between a "meeting" between the two leaders and a summit.

McFarlane said, "It is worthwhile for the two leaders to meet for the purpose of getting to know each other, to hear the other person's priorities, and, as long as no one deludes themselves that such meetings have altered fundamental differences or the depth of disagreement, no harm is done."

He said the meeting could parallel Reagan's session last year with Soviet Foreign Minister Andrei Gromyko at the White House, which followed speeches by both men at the United Nations.

"The president believes that the climate of the relationship is such as to justify a meeting now and for the foreseeable future, absent any dramatic changes in Soviet attitudes," McFarlane said. "There need not be a specific agenda for such a meeting which is oriented in the short term toward the new leaders . . . getting to know each other, surveying the current family of disagreements and assessing each other's commitment to the resolution of problems."

McFarlane said such a meeting was implicit in Reagan's invitation to Gorbachev, carried to Moscow when Vice President Bush attended the funeral of Soviet leader Konstantin Chernenko. He said Reagan also "talked about the imperative of solving problems" and so he has "been positive toward both" a summit and a get-acquainted session.

Reagan "is open to a meeting now and he believes we should press on with an agenda that could lead to a summit."

A senior official said earlier in the week that it was possible a U.N. meeting would produce one or two agreements. McFarlane's remarks today did not mention this possibility.

A meeting between Secretary of State George P. Shultz and Gromyko was announced today in Washington and Moscow, scheduled for May 14 in Geneva, and McFarlane said the discussions will focus on superpower disagreements at the outset and not specifically on a meeting agenda. Those disagreements are in the areas of arms control, human rights, regional conflicts and bilateral issues.

White House officials acknowledged there had been confusion surrounding Reagan's plans for a session with Gorbachev, caused in part by suggestions from senior officials that the administration was insisting on extensive preparations in advance.

They said this was the case for a full-scale summit but wanted to make it clear that Reagan and Gorbachev could meet at the United Nations without such preparations. They said there has been talk for two or three days about the possibility of both a get-acquainted meeting this fall and a more traditional summit later on.

Reagan has not always been willing to have such a get-acquainted meeting. In his Jan. 9 news conference, he said, "To have a meeting, just to have a meeting, doesn't make any sense . . . . I don't think it would make much sense simply to say 'well, now that we're going to talk about these other things at Geneva , let's have a meeting just to get acquainted.' That builds up people's hopes, and some previous presidents have done that and found that the letdown was very terrible."

Meanwhile today, Ambassador Paul H. Nitze, special adviser on arms control, was critical of Gorbachev's announced moratorium on further deployments of intermediate-range missiles.

Appearing on a panel at the American Society of Newspaper Editors convention, Nitze said the Soviet leader's proposal "walks back" from his country's final negotiating position taken in Geneva in 1983. At that time, Nitze said, Moscow was willing to have only 120 SS20 missiles in Europe while freezing the number of those missiles in the Far East, which then stood at 110.

Under Sunday's plan, he said, the Soviets would have 414 SS20s overall and "no constraints" on the number in the Far East.

Gorbachev, he said, was offering a "toughening" of past positions "rather than any iota of concession."

Asked about a Reagan-Gorbachev meeting, Nitze said the "odds are not favorable" that a summit in the near future could produce a breakthrough in the Geneva talks.

Nitze said he expects the Soviets will spend time at the negotiations trying to get the United States to halt its Strategic Defense Initiative, the "Star Wars" research program. Only when Moscow becomes convinced that the U.S. program is going ahead, Nitze said, would its leaders be prepared to negotiate seriously.