The waves of imports that have inundated American shores in the 1980s are helping wash government merger and antitrust policy away from its historical moorings.

For most of the past century, that policy has been based on a belief that mergers that produce giant firms -- or increase the size of existing giants -- threaten to harm consumers and the economy by lessening competition.

The Reagan administration is about to propose a revision of antitrust law, potentially the most important in at least 35 years, that stands the old fear of corporate giantism on its head, according to administration officials.

Under the Reagan administration's antitrust doctrine, corporate size is no longer a stigma.

It may well be a virtue, or even a necessity in today's global competition, officials say, citing the example of the multibillion-dollar Japanese companies whose products are household words in the United States. Many of them were formed through multiple combinations and mergers -- companies such as Mitsubishi, Mitsui and the other great Japanese industrial groups that are descendants of the Japanese "zaibatsu" holding companies that dominated that country's industry before World War II.

America may need its own "zaibatsu," some administration leaders say, and they will campaign for changes in antitrust laws to ease the way for mergers of the largest U.S. companies.

The president's 1986 legislative request to Congress will include five major revisions in antitrust law. Although the details have not yet been released, a central proposal seeks a revision of language in Section 7 of the Clayton Act, amended in 1950, which forbids mergers which "may" substantially lessen competition.

Until the late 1970s and 1980s, giant companies that wanted to merge were usually blocked by the Clayton Act because of their size alone. That policy was chiseled into a U.S. Supreme Court decision in 1963, based on the Clayton Act, which barred the merger of the second- and third-largest commercial banks in Philadelphia on grounds "that competition is likely to be greatest when there are many sellers, none of which has any significant market share."

The Clayton Act "reflects the prevailing view of the 1950s that 'big is bad' while 'small and many is good,' " Commerce Secretary Malcolm Baldrige said last year. "Try to get across that theory to Japan's Toyota Motor Corp. with $20 billion in sales, to West Germany's Hoechst AG with $14 billion in sales, to the United Kingdom's Unilever Group with $20 billion in sales, or to South Korea's Hyundai Group with $9 billion in sales.

"American firms now find that their principal foreign competitors that export to the United States are very large multinational corporations -- not the small or medium sized firms that Clayton's Section 7 sought to preserve in large numbers," Baldrige said.

"Traditionally, Americans have worried about bigness per se," says Fred L. Smith Jr., president of the Competitive Enterprise Institute, a Washington-based interest group that campaigns for a reduction in government regulation of business.

The foreign firms that now provide the toughest competition in many American markets, are not obliged to live under the antitrust restraints that govern U.S. firms. They are free to merge and combine in joint ventures and other partnerships without fear of public and private lawsuits, Smith says. And the result is another major handicap for U.S. firms.

Since the late 1970s, the Justice Department's antitrust division has taken an increasingly lenient view of mergers by major U.S. companies, recognizing the growing significance of foreign trade to the competitiveness of U.S. markets. While the Carter administration blocked a proposed merger between General Electric Co. and Hitachi -- two of the major television producers in the U.S. market, no outside experts expect a government challenge to the pending merger between GE and an even larger competitor in the television business, RCA Corp.

But the administation wants these policy changes written into federal law, where they will govern lower-court rulings and discourage private antitrust suits that threaten "beneficial mergers."

A new version of the Clayton Act's Section 7 would prohibit mergers and acquisitions only if there is a "significant probability" that the merged companies would have the market power to boost prices above competitive levels and keep them there for a significant period of time.

"The revised Section 7 would make sure that the lawfulness or unlawfulness of a merger is based on the real probability rather than a mere possibility, of its having anticompetitive effects," Assistant Attorney General Douglas H. Ginsburg, head of the antitrust division, said in a speech Thursday.

Companies -- even giant ones -- ought to be able to gamble on a merger, the administration maintains. If the result is a more efficient, competitive operation, consumers and society in general will benefit. If the merger doesn't work, the combined company will lose market share and cease to present an antitrust concern, officials say.

In the language of antitrust experts, big companies that want to merge would be able to justify it more easily with an "efficiency" defense -- an argument that the risk of collusion is outweighed by the promise that the merger will produce a more efficient company. In the past, the "efficiency" defense hasn't been much help to big companies seeking to merge.

The president's plan will also include an important specific change to benefit companies that are hard-hit by foreign competition.

Currently, companies can seek import protection from the government if they can persuade the International Trade Commission that they face serious injury from imports.

Under the Reagan proposal, companies that pass that "injury" test would be eligible for an alternative form of protection -- a limited antitrust exemption that would permit mergers, acquisitions and joint ventures for a five-year period without fear of lawsuits by the government or private competitors.

Another legislative proposal would require courts to dismiss private antitrust suits that involve an unreasonable reach of American antitrust laws into foreign markets.

The proposed changes have already triggered a debate, even before they go before Congress.

"There is an important connection between how American companies have been doing abroad and the changes in attitudes toward antitrust enforcement," said Robert Pitofsky, dean of the Georgetown University Law Center and a member of the Federal Trade Commission in the Carter administration.

"What's going on now, however, is that antitrust is being made the scapegoat for a wide range of problems in trade which have nothing to do with antitrust enforcement.

"If American companies are having troubles competing, I don't think it's because they aren't big enough. Making General Motors even bigger isn't going to solve a competitive problem," Pitofsky said.

Basing antitrust policy on a global market perspective "gives too much away," Pitofsky said. The administration is correct in paying more attention to the impact of international competition in judging merger cases, he said. "But they seem to be using international competition as justification for sweeping changes that go far beyond what is required."

Pitofsky and other critics quarrel with the administration's belief that in mergers of big corporations, the gains in efficiency promise to outweigh the risk of collusion or anticompetitive pricing.

"A lot of these mergers have nothing to do with increasing efficiency . . .," Pitofsky said.

Almarin Phillips, a professor with The Wharton School, echoed that concern. "Conceptually, I'm all in favor of having antitrust enforcement pay primary attention to efficiency. At the same time, I'm very uncomfortable with the idea that everything that goes on in the market is prompted by the desire for greater efficiency. I still think we have a lot of market power out there" -- power in the hands of very large companies whose market strength permits them to charge anticompetitive prices or restrict the entry of new competitors. "I think a lot of people who are arguing for more attention to efficiency are blinding themselves to market power problems," said Phillips.