President Reagan today predicted that U.S. relations with the Soviet Union would improve in the long run, but he warned that there may be more hostile rhetoric and little progress in arms control negotiations until the United States begins deploying new Pershing II and cruise missiles in West Germany, Italy and Britain in December.

"Frankly, my own opinion is that the negotiations won't really get down to brass tacks until they see that we are going forward with the scheduled deployment," Reagan told eight American and foreign journalists in a brief interview this morning at the end of the seven-nation western economic summit here.

Asked whether he meant there would be no serious negotiations with the Soviets at Geneva on reducing medium-range missiles in Europe until the new U.S. missiles are deployed, Reagan replied, "Oh, no, we're going to try. The meetings are on now, we're going to try to negotiate.

"I'm just anticipating from the Soviet side, they have based their entire propaganda campaign, everything they've been doing, on seeking to prevent the beginning deployment," he added. "And we have a schedule of deployment, at the request of our NATO allies, and we're going to follow that."

"If there is an increase of tension, it will be the Soviet Union that causes it," Reagan replied to another question about U.S.-Soviet relations. Pointing out that several of the nations represented at the summit here were World War II enemies of the United States, he added, "I couldn't help think several times, 'Why in the world isn't that other so-called superpower, why didn't they have someone sitting at that table able to get along with the rest of us?' "

Asked if he foresaw better or worse relations with the Soviets in the future, Reagan said, "I see better because I think all of us together have a more realistic view of them. Now, this may not be visible in the rhetoric in the immediate future, because there's an awful lot of rhetoric that is delivered for home consumption."

When he was told the Soviets had accused him of "wrecking detente" by pushing through a statement at the summit here supporting his determination to deploy the new U.S. missiles in Europe unless the Soviets reduce their arsenal of similar weapons, Reagan said that "detente, as it existed, was only a cover under which the Soviet Union built up the greatest military power in the world."

The scheduled 30-minute interview was cut in half by White House aides who said an advancing thunderstorm required Reagan's immediate departure if he were to return to Washington by helicopter rather than by car.

French President Francois Mitterrand, West German Chancellor Helmut Kohl, Italian Prime Minister Amintore Fanfani, Canadian Prime Minister Pierre Trudeau, and Japanese Prime Minister Yasuhiro Nakasone also left today. British Prime Minister Margaret Thatcher had returned home Sunday to continue her campaign for reelection there.

Midway through this morning's interview, White House chief of staff James A. Baker III and communications director David R. Gergen came into the sun porch of Providence Hall here to tell Reagan he had to leave.

"I'm sorry, but we've got to go," Baker said. "We're going to have a major problem unless we go."

The president appeared to be in no hurry to leave, and departed from the dictum of White House spokesman Larry Speakes to limit discussion to the summit.

When Reagan was asked whether he saw potential for a superpower confrontation in Central America because the Soviets were increasing aid to the leftist Sandinista government of Nicaragua while the United States supports anti-Sandinista rebels there, Speakes broke in to say, "It is a little off the summit, but if you want to answer it, Mr. President . . . . "

"Well, it is a little off the summit," Reagan said. "I did, in one sesion, simply explain as well as I could the entire situation in Central America. And many of them the other summit participants admitted that they had not been clear on some of what was going on. There has been a step-up of Soviet activity as to bringing in supplies. But we still believe that our plan of economic aid and such military assistance as we think is needed there in the line of supplies--training, mainly--should go forward . . . . We want, indeed, a political settlement if it can be reached."

Later, in a brief encounter with other reporters after returning to the White House, Reagan sought to play down changes he is making among officials handling Central America policy.

The White House removed Thomas O. Enders as assistant secretary of state for inter-American affairs last week, and was reported by senior administration officials to be replacing Deane R. Hinton as ambassador to El Salvador. Enders is being replaced by Langhorne A. Motley, the U.S. ambassador to Brazil; Gerald E. Thomas, the current ambassador to Guyana, is described by the same officials as the leading candidate to replace Hinton.

"These have been played, very frankly, out of all proportion," Reagan said. Contradicting assertions by his own senior officials that the White House is taking a stronger hand with people of its choosing to carry out a tough policy in Central America, he added, "These are changes that don't have anything to do with policy changes or anything else."

Before today's interview ended, Reagan said he had presided over a successful summit that had "not really" changed the opinions of himself or the other leaders on any issue. Yet Reagan appeared more conciliatory toward French desires to improve the international monetary system than he had been before the summit.

The "Williamsburg Declaration" the leaders issued on Monday made a concession to Mitterrand by directing the finance ministers of the seven nations to study ways to improve the international monetary system and to consider the role that might be played "by a high-level international monetary conference."

Reagan said what had happened "isn't so much a modifying of views as it is a learning of what the views really were." He said the high exchange value of the U.S. dollar, which the French consider to be overvalued in relation to the franc and other western European currencies, is "not an unmitigated blessing" for the United States.

"We will have a trade deficit this year of probably $60 billion simply because the high value of the dollar has priced us out of many foreign markets," Reagan said. "We would like to see a better balance."