April 20, 1984

-HOUR RIFFS On Fridays for the last 18 years, doctors, lawyers and business people have been escaping their professional worlds to participate in Jazz at Noon, a lunchtime jam session that now takes place at Freddy's, 308 East 49th Street, east of Second Avenue, from noon to 2:30. The performers, many of whom have played with name bands, always invite a guest star to join them. This afternoon, it will be the singer Maxine Sullivan. There's a $4 charge for listening. It's not necessary to buy food or drink, but Freddy's offers a $7.95 lunch with a choice of four entrees, and drinks start at $3. The number for more information is 888-1633. ELLY STONE'S NEW YORK

The Commerce Department ruled today that Brazil and Argentina have been subsidizing steel exports to the United States. This opens the way to duties that could cripple the nations' sales here and hurt their ability to meet interest payments on their large debts. But the department also announced that it had ended its investigation of related charges by the United States Steel Corporation that Mexico was subsidizing steel exports to the United States. Officials said the case was dropped after U.S. Steel informed the department it would withdraw its complaint because Mexico had voluntarily agreed to limit its steel exports.

The White House rejected Nicaragua's Deputy Foreign Minister as the next Ambassador to Washington at the insistence of the Central Intelligence Agency, Administration officials said today. They said the decision overruled a recommendation made by Secretary of State George P. Shultz and reflected the dominant role the C.I.A. has taken in determining United States policy toward Nicaragua. At the urging of the C.I.A., Administration officials said, the White House is also considering imposing economic sanctions against Nicaragua, including an embargo of banana imports and the cancellation of landing rights in the United States for Nicaragua's national airline, Aeronica. Those measures have been opposed by the State Department, the officials said. A final decision on the sanctions has not been made.

The economy grew at an annual rate of 8.3 percent in this year's first quarter, well above the 5 percent pace in the last quarter of 1983, the Commerce Department reported today. But economists, although surprised at the strength shown by the gross national product, attributed much of the rise to a sharp, unsustainable buildup of the inventories of goods that businesses keep in their factories and warehouses. For the current quarter, many analysts expect a sharp slowing in the growth rate of the G.N.P., which is the total value of all goods and services produced in the country. ''There is less there than meets the eye,'' Martin S. Feldstein, chairman of the Council of Economic Advisers, told reporters. Inventory accumulation accounted for half of the first quarter's increase, he said.

''There goes New Hampshire's ranking as the state with the lowest level of unemployment!'' - Mike Quimby, a carpenter laid off at the Seabrook nuclear plant. (A1:4.)