I have read with interest The Post's reports on the 40th anniversary celebration of the General Agreement on Tariffs and Trade, under which the trading world has prospered since World War II.

The GATT is now in trouble and is threatened by protectionist legislation in the U.S. Congress. But the principal problem of the trading system today is not the agreement itself. Rather, it is the market-sharing arrangements negotiated outside the GATT, primarily by the European Community, the United States and Japan.

It is true that the GATT's rules on agriculture are weaker than those on other products and that they have been compromised by the Common Agricultural Policy of the EC, the 32-year-old U.S. waiver for dairy products and the highly protectionist policies of Japan. (Americans should not forget, however, that the duty-free treatment of billions of dollars of U.S. exports of soybeans to the EC was negotiated under the GATT and is the raison d'etre for recent EC summit consideration of a tax on oil seeds.)

The principal task of the Uruguay Round should be to bring "voluntary" export restraints and other market-sharing deals under the GATT and to strengthen the rules on agriculture, particularly on subsidies. Also needing priority attention are the proliferation of EC preferential trading arrangements and the unwillingness, so far, of the developing countries, especially the newly industrialized countries, to assume obligations comparable to their increasing trade importance.

This is not to say that the "new" issues of services, intellectual property and trade-related investment should not be dealt with. On the contrary. But negotiated rules in these areas will be of little benefit if respect for and improvement in rules on traditional subjects are not forthcoming.

WILLIAM B. KELLY

Washington The writer was deputy director general of the GATT from 1980 to 1987.