W. Averell Harriman, recalling that "I knew Trotsky, but I didn't know Lenin," drew on half a century of experience with the Soviet Union yesterday to testify in favor of the new strategic arms limitation treaty.

Harriman's appearance was the theatrical high point of the Senate Foreign Relations' hearings on SALT.

The former governor, cabinet minister and ambassador had to read his prepared statement with a magnifying glass, and asked the Senators to speak up: "I'm a little deaf - I'm not only a little deaf, I'm very deaf." But he made his points with conviction and a strong voice, and answered questions with intellectual and political agility.

Sen. George McGovern (D-S.D.) observed that the rulers of the Soviet Union are "men of your age," but that was more a compliment than an accurate remark. Harriman, 87, is 15 years older than Leonid I. Brezhnev, the ailing Soviet leader.

Harriman told the Foreign Relations Committee there was nothing about the soviet system that he liked, but he nevertheless was certain Soviet leaders were interested in avoiding war and in arms control agreements. "They are concerned about their survival, just as we are," Harriman said.

Asked if the United States can trust the Russians, Harriman replied: "You can trust the Russians to do what they think is in their interest," and controls on nuclear armaments meet that test.

Recallins his personal involvement in the history of the nuclear age, Harriman noted that in 1963, " khrushchev was prepared...to agree to a complete comprehensive test ban" - that is, a ban on all nuclear tests, above or below ground. Negotiations for such a ban broke down after the United States demanded seven "on-site" inspections to police the proposed ban, and the Soviets offered three.

"Think how greatly the whole development of nuclear weapons would have been restrained by the prohibition of testing," Harriman said, noting that the most dangerous weapons in today's arsenals came much later.

Harriman said approval of SALT II by the Senate will "tend to strengthen the more reasonable group" inside the Soviet politburo, but defeat "might lead to a hard-line succession" to Brezhnev.

Pressed about this by Sen. Paul S. Sarbanes (D-Md.), Harriman said he would not make a firm prediction about the soviet reaction to a Senate vote against SALT. "I fear it greatly," he said. "It will strengthen the hard-liners." But he declined to predict an irreparable break in Soviet-American relations. "I never give up on anything," Harriman observed. "I would be willing to go to Brezhnev to talk to him about picking up the pieces that were left."

Sen. Jacob Javits (R-N.Y.) asked Harriman if a senator who genuinely felt that U.S. security interests would be damaged by SALT II should vote against it.

"I cannot answer your question," Harriman said, adding that such a conclusion was beyond his imagination. "If anyone has that feeling," he told Javits, "he's misinformed."

The committee also heard testimony from two former under secretaries of state, George Ball and Eugene Rostow. Ball said the ATNO allies wanted SALT II approved; Rostow said they would welcome its rejection if that was accompanied by an aggressive new American foreign and defense policy. CAPTION: Picture, Harriman uses magnifying glass to read testimony before the Senate Foreign Relations Committee. AP