Third-world states spending less on arms in recent years. New suppliers overtake some traditional dealers

For now, the third-world arms race is slowing down. In 1987, spending by underdeveloped nations on new weapons from abroad was only about half of what it was in 1980, according to an authoritative new congressional study.

Declining oil revenues, increasingly tight budgets, and the need to absorb already purchased weapons are among the reasons for this drop. But in two nations locked in a fierce war - Iran and Iraq - demand for weapons has remained strong.

``The total value of arms transfer agreements with Iran and Iraq ... constituted over one-fifth of all agreements by all suppliers with the third world,'' says the study by Library of Congress analyst Richard F. Grimmett.

The Soviet Union is far and away the largest supplier to the third-world arms bazaar. Last year, Soviet sales accounted for almost half the value of all arms agreements with underdeveloped nations. Since 1980, Soviet weapons deliveries have included 7,390 tanks and self-propelled guns, 13,115 armored cars, and even 13 submarines, Mr. Grimmett says.

The United States is a distant second in the supplier race, accounting for 19 percent of third-world arms traffic in 1987. The US has sold 3,670 tanks and self-propelled guns, 6,947 armored cars, and zero subs to underdeveloped countries in the last seven years.

Further down the list, ``certain emerging suppliers of armaments to the third world have ranked ahead of some of the traditional, industrialized suppliers,'' reports Grimmett. China, for instance, is now the fourth largest dealer of weapons to the third world, ranking ahead of West Germany and Italy. Czechoslovakia and Brazil have also made substantial sums as arms dealers.

Iran is one of China's best customers, according to Grimmett, whose study contains substantial amounts of previously classified information.

With many over-the-counter arms sources closed to it, Iran has purchased 15 percent of its arms since 1980 from the Peking government. Western Europe is estimated to have clandestinely provided about 30 percent of Iran's weapons for the period, and East-bloc countries about 33 percent.

Iraq, on the other hand, overwhelmingly depends on the Soviet Union for its military goods. Some 47 percent of its armament this decade has come from the USSR, with China and Western Europe providing significant but smaller slices.

The Near East and South Asia, with their relative wealth and many regional rivalries, are together still the largest third-world arms market. Over the last three years, the countries of these regions have accounted for almost 70 percent of the deadly arms purchased by underdeveloped nations, according to Grimmett's figures.

But their lead in this race, doubtless a dubious distinction, is slipping. While the Near East-South Asia share has shrunk in recent years, both sub-Saharan Africa and Latin America have increased their weapons purchases.