THE SALES TRIP to Japan by Juanita Kreps, the Secretary of Commerce, has its comic aspects. She's serving as tour guide to large delegation of American business people in the hope of increasing this country's exports to Japan. Is the federal governement trying to teach the standard-bearers of the free-enterprise system how to sell? Mrs. Kreps must feel as though she were offering swimming lessons to a flock of reluctant and suspicious ducks.

But unfortunately those ducks have reason to be cautious. It's very cold water, and there have been some mysterious disappearances in that pond. Successful or not, Mrs, Krep's mission suggests the necessity of political intervention in selling to Japan. The troubles of U.S. exporters have become much more than a narrow commercial matter. Because of the effects on jobs and currency values, the huge American deficits in U.S.-Japanese trade have created serious political strain. Last year the deficit was $8 billion, a figure that everyone agreed was too big.The Japanese government predicted that it would decline this year. Instead, it is rising to somewhere around $14 billion.

The increasing value of the yen and the decline of the dollar should have cut Japanese sales here, according to economic theory, and improved the sales of American products there by making them cheaper. But that doesn't seem to be happening. Perhaps the experience of the Zenith Radio Corporation offers a fragment of explanation.

Zenith is in ferocious competition with Japanese television manufacturers in this country. For nearly a decade it carried on litigation charging the Japanese with dumping - that is, illegally selling here below their prices at home. Zenith lost in the Supreme Court last summer. The company is not unbiased. But neither is it uninformed.

Five million color television sets were sold in Japan in 1976, according to John J. Nevin, the chairman of Zenith, and 452 of them were imports. Because of the exchange rates, Mr. Nevin says that he could under-sell the Japanese makers by wide margins in their own markets - if he could get into the market. The reason are always vague and polite, he says, but they add up to no access. Regulatory inspections are interminable. Wholesalers are slow and retailers are habit-bound - precisely the opposite of the sharp performance of the same companies on the export side of the equation.

Until the 1970s, Japan seemed a rather poor market and, anyway, the overvalued dollar put American manufacturers at a price disadvantage there. The U.S. government tolerated a good deal of discrimination against American goods, because it was trying to encourage economic growth abroad. Now Japan is suddenly faced with the consequences of its new wealth, and the adjustment is coming unmanageably fast.

Because Japan ran big trade surpluses, the value of the yen rose. Last year alone, it went up 38 percent against the dollar. To return to television sets, imports now represent a terrifying threat to Japan's electronics industry. Zenith's price, on the Japanese market, would be a third lower than last year. Japanese makers would either have to cut prices wildly or be undersold by vast margins. American officials fear that the government in Tokyo may keep stalling indecisively on imports while the yen floats higher still. That's why Mrs. Kreps went to Japan.