President Reagan's national security affairs adviser, Robert C. McFarlane, offered a deeply pessimistic view today of the prospects for a U.S.-Soviet agreement to slow the arms race.

McFarlane offered his assessment in a speech here as the White House announced that Reagan's meeting with Soviet Foreign Minister Eduard Shevardnadze has been set for Sept. 27. The meeting is expected to "review all areas of our relations and help prepare" for Reagan's November summit with Soviet leader Mikhail Gorbachev, White House spokesman Larry Speakes said.

In a speech to the Santa Barbara Channel Club, McFarlane cited several examples of what he described as Soviet decisions in recent years to accelerate superpower competition in chemical weapons, ballistic missile defenses and intermediate-range missiles.

In each case, McFarlane said, the Soviets attempted to "resume or initiate competition in an area where there hadn't been any at all."

McFarlane pointed to the U.S.-Soviet charges and countercharges over ballistic missile defense, including Soviet complaints about Reagan's proposed "Star Wars" nuclear shield.

The Soviets, he said, have proposed "something we believe is nonnegotiable and nonverifiable -- a ban on research even as they pursue the largest research program on Earth." McFarlane described as a "masterpiece of chutzpah" the Soviet claim that the U.S. program is designed to acquire a first-strike capability.

"In short, we're having a lot of trouble establishing a real dialogue," McFarlane said.

Reagan is willing to meet the Soviets "halfway in developing responsible solutions to outstanding problems," he added.

"But without some change in the Soviet approach to security issues, in fact in the thinking that underlies it, I fear that even incremental improvements will be extremely hard to reach. And they will be much less likely to gather momentum, to build on each other."

McFarlane also noted Soviet support for Cuba and Libya and the Soviet invasion of Afghanistan. In Cuba, he said the Soviets had "reignited a source of conflict" in the region. Although the United States can "deal with these difficulties when they are placed in our way," McFarlane said, "it certainly sends us loud messages that can't be ignored about the motivations of Soviet policy."

"It makes improvements in other areas more difficult. It all but guarantees that any small steps forward that we may be able to take will be isolated, hard to preserve, and perhaps devalued in advance by both sides," he said.

Although such political and military issues have dominated the agenda of U.S.-Soviet relations, he said the "most momentous changes" could take place in human rights.

"The most durable and far-reaching kind of improvement in Soviet-American relations -- and probably in the Soviet Union's relations with almost every country of the world -- would be created by events inside the Soviet Union," he said.