TOKYO, JAN. 12 -- An organization representing Japanese farmers has decided to reduce its imports of American feed grain by one-fifth in retaliation for U.S. efforts to open the Japanese market to other farm products, organization officials said today.

The organization, the powerful Central Union of Agricultural Cooperatives, will buy 1 million metric tons of corn and other grains from Argentina or other nations instead of from the United States, even though the cost is likely to be higher, the officials said.

The announcement represents the first such overt retaliation by the Japanese in an increasingly bitter trade war between the two nations, officials in Tokyo said.

"If the United States sticks to its high-handed attitude and tries to destroy our longstanding friendly bilateral relations in disregard of the voices of many Japanese, we shall have to take countermeasures," said Mitsugu Horiuchi, president of the organization.

Horiuchi spoke at a rally of about 3,000 farmers and supporters protesting U.S. policies in Tokyo on Monday. The senior executive director of his organization, Iwao Yamaguchi, confirmed today that he had taken the first concrete steps toward reducing reliance on U.S. farm products, initiating talks with Argentina, China and Australia to buy 500,000 metric tons of corn.

Yamaguchi said he could not estimate the dollar value of the punitive action, but it is likely to be in the hundreds of millions of dollars.

Japan is the single largest customer for U.S. farm products, but it also restricts imports of some items that would compete directly with Japanese agricultural production. The United States, suffering from a $60 billion trade deficit with Japan, has been pressing Tokyo to loosen those restrictions.

The international trade body known as the General Agreement on Tariffs and Trade ruled late last year in favor of the United States on 12 types of food products and processed agricultural goods, saying Japanese import restrictions violate international agreements. But Japanese farmers, an influential lobby inside the ruling Liberal Democratic Party, have been urging the government not to accept the GATT decision.

The GATT decision covered milk and cream products, processed cheese, dried peas and beans, sugars and syrups, ketchup, tomato juice and other products. Washington is pressing separately for Japan to allow more imports of U.S. beef and citrus products.

U.S. officials in Tokyo said they would not comment on the farm group's announcement, especially since the action does not represent a governmental decision. The U.S. Feed Grains Council in Tokyo told Japanese television that the decision was "baffling" and would "send the wrong signal to Congress."

The Japanese farm lobby argued that giving in to U.S. pressure would put many Japanese farmers out of business. The cooperatives' group, which handles 40 percent of all Japanese agricultural imports, said it is determined to send a signal to the United States.

"The price may go up," Yamaguchi said. "Now that Japanese agriculture is in danger, and it's an emergency, we have to take emergency measures, even if the price goes up a bit."

Japan last year imported 21 million metric tons of feed grain -- worth $2.3 billion -- that was used to feed chickens, cows and other farm animals, according to Japanese statistics. More than half came from the United States.

The cooperatives' trading group imported about 4.5 million tons of grain from the United States. Yamaguchi said that figure will be reduced by 20 percent in retaliation for U.S. pressure.