A week ago, Californian Dr. Jerry Jampolski shot off a Telex to friends here, wishing them "peace and love" and abruptly cancelling his long-held plans for a summer trip to the Soviet Union.

For Jampolski's Moscow friends, no explanation was needed.

Last Saturday, the night bars at the Cosmos, the national and the Intourist hotels, Moscow's answer to the M Street strip -- usually bustling havens for American tourists -- were nearly deserted.

"And you know why," a Cosmos bartender told an inquisitive patron.

Before the Chernobyl nuclear disaster two weeks ago, travel agents were billing the Soviet Union as the American vacationer's great escape from the terrorist threat and the weak dollar in Europe. An upbeat trend in U.S.-Soviet relations, buoyed by the Geneva summit last November, heightened expectations here for 80,000 American visitors, a 1980s record.

After the 1979 Soviet invasion of Afghanistan, the 1983 shooting down of a Korean airliner and other ruptures in the U.S.-Soviet relationship had led to a general downturn in visitors from the United States, this was supposed to be the year that "Americans started coming back to Moscow," a U.S. official here said.

Pan American airlines geared up with regular flights to the Soviet Union, the first U.S. air link since the invasion of Afghanistan. And the Goodwill Games in the Soviet capital this summer, promising the biggest international athletic competition since the Los Angeles Olympics, added to the luster of a whirlwind tour of the Soviet Union.

"Now," New York tour guide Andrew Harkushka said in an interview here, "tourists see Chernobyl as a good excuse to see America first."

After the April 26 nuclear explosion and fire, cancellations came quickly from all across the West. Last Saturday a London ballet company, worried about the threat of radiation leaks from Chernobyl, backed out of a tour scheduled to start here next week. A British Broadcasting Co. film crew, assigned to Moscow to shoot a documentary, also balked and refused to come.

"Don't go. Don't go," friends told Harkushka before he left New York last week.

Dozens of U.S. tour groups have heeded the warning. Tour Designs, a Washington company specializing in Soviet trips, reported three large group cancellations. Said Robin Royle, a Tour Designs representative: "With so much confusing information, people are deciding better to be safe than sorry."

Pan Am jets, which started arriving in the Soviet capital the day after the nuclear disaster was first announced, have been averaging less than 20 percent of their passenger capacity, according to Jennifer Young, the airline's Moscow representative. Arriving British Airways and Lufthansa passengers, too, have reported "lots of rows of empty seats."

"People may be playing it cautious," Young said in an interview. "They want to see what kind of information develops." But U.S. officials here think that the effect on U.S. tourism in the Soviet Union for the rest of the year is likely to be "significant."

Soviet officials have met the worries in the West with a mixture of complacency and defensiveness, provoking a deeper ire among U.S. tourists and tour guides here.

The official Soviet media at first mocked the tourists' fear of radiation. Soviet television showed British tourists standing in front of the Kremlin, complaining about their own tour guides' general precautions in the wake of the Chernobyl disaster.

Despite the health warnings Ukrainian officials have issued in Kiev, the Soviet travel agency Intourist has said the accident required no changes in planned "tourist tours."

So far, the U.S. State Department travel warning has extended only to the region adjacent to Chernobyl, including Kiev, the Ukrainian capital and third largest Soviet city, 80 miles away. Moscow and Leningrad are not included. Unlike some other western embassies here, the U.S. Embassy has not issued a warning against consuming milk in the Soviet capital.

Western analysts in Moscow doubt that the damage to tourism in the Soviet Union will significantly drain the country of hard currency, only a fraction of which comes from tourism.

Pan Am and U.S. tour operators are holding out hopes that publicity about the Chernobyl incident will fade and that tourists' concerns will fade with it. But one U.S. travel agent pointed out that it took two years to recover from the drop in tourism following the Soviet destruction of the Korean airliner in September 1983.