As recently as two years ago, a bridge across the Mississippi River was about as American as apple pie and the Fourth of July. Now it is likely to be made in Japan.

The newest Mississippi River span is soon to be fabricated in Japan at a cost of $42 million, one-third less than the lowest American bid, and floated by barge in pieces the size of ship hulls to its destination at Luling, La. American bridge builders claim they've been sold down the river and they're mad.

Halfway around the world from Japan, American pharmaceutical manufacturers are caught in a bind of a different sort. Imported drugs can't be sold in France unless they are manufactured under supervision of French inspectors, but French inspectors can't do their supervising outside the borders of France - "le Catch 22," one trade official calls it.

The import-threatened bridge builders and export-thwarted medicine makers are among thousands of corporations, unions, trade associations and farm groups - to say nothing of lawyers, lobbyists and consultants - that have high stakes on the table of the international trade talks now approaching a climax in Geneva.

Even those whose eyes glaze over at the mention of countervailing duties, orderly marketing agreements, nontariff barriers and safeguard escape clauses will be affected, like it or not, by this latest round - the so-called Tokyo round - of trade bargaining.

The biggest stakes may involve the giant multinational corporations that fuel their operations with enormous profits drawn from the world's $1 trillion a year in trade. But the ripple effect from trade shifts touches nearly everyone, affecting jobs, prices, living standards and the availability of consumer goods, from autos to zippers.

Not only does trade create and destroy jobs, but trade imbalance can trigger inflationary waves throughout the industrialized world, as it learned painfully from Middle East oil producers over the past decade. The politicians who are negotiating the agreement will be helped or hurt politically by the economic fallout from its implementation. This is true of President Carter, and of the leaders of Japan and European nations that depend even more heavily on trade than the United States does.

C. Fred Bergsten, assistant secretary of the treasury for international affairs, put the U.S. argument for trade this way recently:

"One out of every five jobs in this country now produces for exports to other countries. One out of three dollars of corporate profit in this country derives from international activities of American firms."

The United States not only exports about one-fifth of all its manufactured goods, about $76 billion worth, but about one-third of its total agricultural production abroad, roughly $24 billion. Foreign sales account for 45 percent of wheat production, 60 percent of soybeans, 57 percent of milled rice.

While thousands of American jobs are threatened every year by a rising volume of low-cost imports, throwing a panic into industries ranging from shoes and apparel to steel and electronics, the United States remains the world's largest exporter as well as its biggest importer.

Although exports have been sluggish over the past two years, they represent a vastly bigger share of U.S. production than they did just 10 years ago.

Until the late 1960s, trade accounted for between 3 and 4 percent of the nation's gross national product, which measures the total production of goods and services over a year's period of time. Now it is 6.2 percent for exports, 7.8 percent for imports.

Trade's real importance is seen if it is measured solely against total production of goods alone, as opposed to goods and services combined. Ten about 12 percent of all agricultural, mining and manufactured goods produced in this country. In 1976, the latest year for which the Commerce Department has compiled such statistics, exports accounted for 22.8 percent of domestically produced goods, down somewhat from ahigh of more than 24 percent in 1974.

But the other side of the coin is that imports have been rising even faster. By 1976 the United States was importing an amount equivalent to 24.2 percent of domestically produced goods, a larger share than it was exporting.

In raw figures, the country 10 years ago was selling $30 billion worth of goods abroad and buying $26 billion, for an overall trade surplus of about $4 billion. By last year, it was selling $121 billion and buying $147 billion, producing a trade deficit of nearly $27 billion, a staggering increase over the $6 billion deficit of the previous year.

Moreover, imports of machinery, transportation equipment and manufactured goods rose dramatically, suddenly overtaking oil as the biggest component of the country's trade deficit during the first five months of this year. A report by the National Association of Manufacturers pointed out that the traditional U.S. surplus in trade of manufactured goods declined to the point of a $3 billion deficit in the first quarter of 1978.

The trade imbalance was one of many factors that led the Carter administration, in the person of Special Trade Representative Robert S. Strauss, to breathe new life into trade talks that had been sputtering along inconclusively since they opened in Tokyo in 1973.

Ninety-eight countries, including most U.S. trading partners, are participating in the talks, now being conducted in Geneva. More than 5,000 separate trade items are on the U.S. shopping list alone.

The overall purpose of the talks is a reduction in worldwide barriers to imports and exports, including both tariffs and such nontariff barriers as product standards that discourage imports, government subsidies that encourage cut-rate exports and government agency purchasing practices that effectively preclude foreign competition.

It is these nontariff barriers - such as the French inspection requirement for imported medicines and a broad array of Japanese policies that produces cut-rate bridges for the American market - that have been the principal focus of the Geneva talks.

While it has been generally expected that industrial tariffs would be cut by up to 40 percent over the next decade, with some labor-sensitive items excluded or treated separately, crucial details were still taking shape as broad outlines of the package were being put together for presentation to the Bonn economic summit this weekend. U.S. trade negotiators hope to have a final agreement ready for action by Congress early next year.

Agriculture has been a major stumbling block. Strauss warned recently that there can be no overall trade agreement unless Japan and European countries open their doors to more American farm products.

A healthy trade balance in farm products is sought not only to offset deficits in oil, machinery and manufactured goods but also to ease the way for approval of the agreement by Congress next year.

One major problem - both for the negotiations and for the eventual congressional action - is a rising tide of protectionist sentiment, both here and abroad, that stems from slow growth, high inflation, serious unemployment in many foreign countries and other economic traumas.

Pressures to return to economic isolationism are strong, Strauss has warned, and "cannot be contained much longer."

In this country, the evidence can be seen almost daily: 20 steel-state senators warn that the trade agreement will be in trouble unless a separate steel pact is negotiated to restrict export subsidies and below-cost dumping of exported steel; Irving Shapiro, chairman of DuPont and head of the powerful Business Roundtable, calls for legislation to exempt textiles from any Geneva tariff reduction agreement: state after state passes "Buy America" legislation to keep foreigners from getting government business and similar proposals for the federal government are piling up in Congress.

Makers of textiles and apparel, steel, shoes, industrial fasteners and televisions and their related unions have been among the most visible protesters, but there is a "creeping paranoia" even among some historically free-trade industries, according to Harald B. Malmgren, a former U.S. deputy trade negotiator now in private consulting work. Some big multinationals "see their home base getting murdered and they want to constrict to protect it," Malmgren said recently. Even highly sophisticated industries like computer manufacturers see Japan just around the corner, he noted.

Raymond Garcia, vice president of the Emergency Committee for American Trade, a group of 63 large multinationals that lobbies for liberalized trade, noted the case of Allis-Chalmers, a big machinery exporter.

While the firm as a whole supports relaxation of trade barriers, its hydroturbin division - closed out of Japanese markets but forced to compete with Japanese products here - testified recently before Congress in favor of expanded "Buy America" protections.

The conversion from free trade to protectionism has been particularly dramatic in the case of the AFL-CIO, whose workers have been hurt by job losses tied to trade imbalances. Said AFL-CIO research director Rudy Oswald in a recent article: "The old doctrine has lost its premises . . . and new threories have not yet emerged to accomodate the overwhelming facts of state enterprises and trading monopolies, massive subsidies and the dissappearance of true cost pricing, managed economies and currencies and a multiplying host of nontariff barriers and inducements."

Strauss' strategy, according to one observer, has been to put out one brushfire after another before they coalesce into an all-consuming blaze - to prevent the amassing of a broad-based protectionist coalition. But economic conditions appear to be conspiring to make the job more difficult. "Even Bob Strauss has his limits," said one of his trade expansionist allies.

One problem is that the benefits of trade are diffuse and often long-range, while the problems are visible and immediate. "The companies that are too busy to come to Washington to doing well are busy doing well and lobby," said on observer.

An irony is that protectionist pressures, born of real threats to major industries and their workers, are rising at a time of unparalled American dependence on foreign trade.

"What it means is that after all our talk about interdependence, we're now experiencing it," says Malmgren, "and the experience is traumatic."