Contacts between private U.S. and Soviet citizens once were stiff, tightly controlled encounters marked by suspicion if not outright hostility. Yet today, as Mikhail Gorbachev prepares for his second trip here, they have become one of the hottest growth industries on the international scene.

The overall number of Soviets visiting the United States doubled from 1984 to 1987 and then increased fivefold from 1987 to 1989 to nearly 60,000 a year. At the same time, U.S. travelers to the Soviet Union nearly quadrupled, from 35,058 to 136,210.

The number of private, nonprofit organizations involved in joint U.S.-Soviet activities has nearly tripled in the past seven years, according to the Institute for Soviet-American Relations, a private group that monitors non-governmental exchanges between the two countries. There were 131 such organizations listed in the institute's 1983 handbook. The number grew to 232 by 1986 and to 313 in the current edition.

Moreover, the interaction is no longer limited to the usual world of artists, scholars and educators involved in general friendship exchanges. It now includes intensely political activities aimed at specific policy goals within Soviet society in health, trade, economics and, particularly, the environment.

B'nai B'rith International, a Jewish social service organization, recently opened four regional offices in the Soviet Union.

A Latvian environmental group tests produce for radioactivity in the central market in Riga with five Geiger counters it obtained from its Canadian chapter.

And the Natural Resources Defense Council, working closely with Soviet environmental activists, announced earlier this month that Soviet public pressure forced Occidental Chemical Corp. "to postpone plans for a proposed $200 million polyvinyl chloride production plant in the Ukraine."

Contacts between U.S. and Soviet private organizations, greatly aided by improved telephone communications and the advent of the facsimile machine, are out of control. At the same time, official governmental contacts on a widening spectrum of issues has increased to the point that it's getting hard for the Bush administration to keep up.

Deputy national security adviser Robert M. Gates called a meeting of about two dozen senior administration officials earlier this month to discuss "departments conducting independent activities with the Soviet Union," according to a source who was present. The point of the meeting, the source said, was to have departments "coordinate and clear things" through the State Department so that State would know what everyone was doing. Each participant was to give a two- or three-minute presentation on the nature and degree of his department's or agency's contacts with the Soviets.

Many of the private programs began just a few years ago with the limited goal of simply having Americans and Soviets meet each other. In the last two years they have dramatically altered their focus.

"We've gone from programs getting citizens together to this year developing substantive programs trying to address the urgent needs of the Soviet Union," said Dale Needles, vice president of the Center for U.S.-U.S.S.R. Initiatives in San Francisco.

The 7-year-old organization, with a dozen full-time staff and volunteer field coordinators in eight Soviet republics, uses daily computerized electronic mail messages to its Moscow and Leningrad coordinators who maintain contacts with the other coordinators.

The center has programs to develop small businesses in the Soviet Union, to combat alcoholism and to promote private clothing production and "bio-intensive gardening" to boost agricultural production, Needles said.

The Natural Resources Defense Council is now working with a coalition of Soviet environmental groups "trying to build up their capability to work on environmental programs, giving them information and technical support," according to NRDC attorney Jacob Scherr.

"Four years ago it would not have been possible to work with Soviet environmental groups," he said. "Now the movement in the Soviet Union involves hundreds to a thousand groups. We're expanding, we're interested in working with every level of society. . . . We're trying to empower the public to be involved to force government to end pollution.

"It's pretty amazing. The change has been tremendous," he said.

No organization could be more amazed than B'nai B'rith International. "We used to demonstrate in front of the embassy," said Buzzy Gordon, a spokesman for the group. "Now we get invited to their receptions and they are invited to our symposiums."

"The vigil still goes on" in front of the embassy, he said, because some Jews are still refused permission to leave. "But even so," Gordon said, "we are very much dealing with the Soviet Union, with the human rights bureau of the foreign ministry and we maintain close contacts with the Soviet Embassy."

The expanding contacts have become almost routine in some places.

When a dozen Soviet teenagers showed up in southern Maine in 1987 to attend the Samantha Smith Worldpeace Camp, "that was a major groundbreaking event," said Jane G. Smith.

The camp is named for her daughter, who gained worldwide fame when she accepted a personal invitation in 1983 from Soviet leader Yuri Andropov after she wrote him of her fear of a nuclear war.

She died, at age 13, in a plane crash two years later.

"We were very careful that first year about security," Smith said. "We contacted the sheriff's office, we notified the State Department. There were still people around who thought we shouldn't talk to them, let alone be friendly," she recalled.

"Now it seems there is a Soviet on every block and people love it. It's definitely not like it used to be. We no longer handpick Americans to be with them. We don't do anything like notify the State Department and local authorities."

Smith estimated there will be about 350 Soviets age 12 to 16 attending summer camps in the United States this summer, but added that it's hard to keep track. "Now every other week I hear of another group."

The dramatic expansion of U.S.-Soviet exchanges is reflected in the boom in Soviet cities signing up with Sister Cities International. "At the end of 1986 there were six U.S.-Soviet relationships," said Sister Cities President Thomas W. Gittins. There are now 55, he said, and that number will nearly double next year.

"Our relationship with our counterpart {organization} in Moscow is as active as with any" other country, Gittins said, with "almost daily faxing back and forth."

Cincinnati's project with its sister city, Kharkov, in the Ukraine, shows how expansive some programs can become. The project includes reciprocal visits of city officials, language professors and students.

But it also joins five local architects with counterparts from Kharkov who will jointly design a project to be built in each city. Newspapers in each city will publish monthly columns written by a reporter from the other. A joint retail venture will sell Cincinnati products in Kharkov for rubles and Kharkov products in Cincinnati for dollars.

And Cincinnati, reflecting the communications explosion between the two countries, plans to install Kharkov's first fax machine, "to permit daily instant written communication," according to Cincinnati project director Joseph J. Dehner.