Eugene V. Rostow, the director of the U.S. Arms Control and Disarmament Agency, said yesterday it is "plausible" the United States might use nuclear weapons first in a conflict with the Soviet Union.

Rostow, appearing on "Meet the Press" (NBC, WRC), said the United States' NATO allies "are very firm, extremely firm on that very subject" because their security depends on the "implicit threat" that nuclear weapons would be used "to prevent them from being overrun, even by tanks."

In a speech before the United Nation's special session on disarmament last week, Soviet Foreign Minister Andrei A. Gromyko relayed Soviet President Leonid I. Brezhnev's pledge not to use nuclear weapons first in any conflict. But the United States rejected Brezhnev's request that it make a similar promise.

Shortly after the speech, the State Department issued a statement calling the Soviet pledge and challenge "not an effective way to reduce the danger of nuclear war." The State Department referred to an April 6 speech by Secretary of State Alexander M. Haig Jr. who said that the threat of nuclear retaliation is important to the defense of Europe because the North Atlantic Treaty Organization is outnumbered in conventional troops and weapons by the Soviet's Warsaw Pact forces.

Haig argued that renouncing first use of nuclear weapons would require the West to maintain the same levels of ground forces as the communist bloc, and could require the United States, in particular, to resume the draft, triple the armed forces and adopt a wartime economy.

When pressed on the plausibility of the theory that the United States would actually attack the Soviet Union with nuclear weapons if Soviet tanks rolled into Germany, Rostow said, "The question of uncertainty about that response has been the main element of deterrence now for a generation."

Rostow said he thinks the pacificist movement in Europe is "much weaker, and all my European friends tell me it's much weaker today than it was six or eight months ago" before President Reagan and other U.S. officials began responding to public concerns.

Rostow also said that while the people behind the worldwide antinuclear movements are "sincere" and "deeply concerned," there is "no doubt" the communists are attempting to exploit and manipulate them.