How bad will the euro slump get? On Wednesday, the beleaguered currency hit a four-year low of $1.21, on nagging worries that the Greek debt crisis will continue to drag down the rest of the euro zone economy-and possibly spread beyond. By Friday the euro had rebounded to nearly $1.26, but it's nonetheless down 13% this year.

The situation has investors wondering whether to overhaul their stock portfolios to protect against a long-term euro plunge. They may be overthinking, especially if they already hold a big slug of U.S. multinationals.

The classicresponse to a sinking euro would be to sell shares of U.S. manufacturers that sell in Europe and buy shares of, say, German, French and Dutch competitors that sell in the U.S. The weakening euro, after all, makes their goods cheaper in the U.S., boosting sales and market share.

But in recent years many big corporations have taken great care to diversify their operations globally, in part to minimize the effects of currency swings on their profits. Investors wary of currency fluctuations 'should invest in multinational companies, which earn money around the world,' advises David Bianco, chief U.S. equity strategist at Bank of America Merrill Lynch.

One such multinational, Peoria, Ill.-based Caterpillar, a maker of construction and mining machines and one of the nation's largest exporters, has manufacturing plants worldwide, including in euro zone countries like Germany, France and The Netherlands.

Only a relatively few of the company's machines, most of which sell in low volumes, are made exclusively in the U.S. One example: giant trucks that can carry up to 380 tons of debris are produced in Decatur, Ill., about 80 miles from the company's headquarters. But smaller, higher-volume machines and parts are made around the world, often close to where they're sold, says Caterpillar spokesman Jim Dugan.

Mr. Bianco recommends investors look at regional sales to determine how evenly companies' currencyexposure is spread. Costs matter, too, but most multinationals try to disperse costs and sales evenly in the regions in which they operate, he says. U.S. sales contributed just 31% of Caterpillar's total last year, while customers in Europe, Africa and the Middle East bought the largest share of exports-40%.

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The euro decline poses little problem for Caterpillar, says Robert McCarthy, a senior analyst with Robert W. Baird & Co., because many of the machines it builds in the U.S. and exports are ones for which it faces limitedcompetition. Shares of Caterpillar are up 7% this year, versus a 2% decline for the Dow Jones Industrial Average.

Other U.S. companies spread their sales even more finely by region. Coca-Cola last year did business using 71 currencies besides the dollar. 'It's hard to find a more geographically diversified company,' says Philip Gorham, a stock analyst with Morningstar Equity Research. Over the long term, that protects the company from currency swings, he says.

In recent years Coca-Cola has seen rapid sales growth in emerging markets, so it plans to invest $2 billion to expand its distribution in China over the next three years. Its shares have fallen 9% this year, but they carry a 3.4% dividend yield, and the company has increased payments for 48 straight years.

Some multinationals are diversified by product line as well as geography. Based in St. Paul, Minn., conglomerate 3M has a hand in everything from touch screens to dog chews. Last year it collected 37% of its sales in the U.S.; 27% in Asia Pacific countries; 26% in Europe, the Middle East and Africa; and the rest in Latin America and Canada. Its stock is down 1% this year, a smidgen better than the Dow.

'They make in a region most of what they sell in a region,' says Nicholas Heymann of investment bank Sterne Agee.

What's more, 3M now generates so much free cash that it can reinvest it at a rising rate in emerging markets to bolster future growth there, according to Mr. Heymann.

'They have an opportunity to take the 10% to 20% market shares they have for products in emerging markets,' he says, 'and expand them to the 40% share they enjoy on some of the same products in developed markets.'