After 39 years promoting Washington business, Clarence Arata yesterday said he will retire as executive vice president of the Metropolitan Washington Board of Trade.

John R. Tydings, who is assistant executive vice president, will take over Jan. 1 as chief executive of the Greater Washington business association.

Arata joined the board in 1938 as assistant manager of its convention and visitors bureau and had been executive vice president since 1971.

Long active in bringing conventions to Washington, Arata said yesterday he hopes to see that work completed early next year with congressional approval of the Mt. Vernon Square Civic center.

Arata has been the Board of Trade's leader on the convention center and said he will continue to work on that project when he officially give up Board of Trade duties.

He said the convention center would "increase the city's posture as a convention city," noting that such gatherings here already have made Washington "one of the major tourist attractions and one of the most important cities of the world."

After retiring from the Board of Trade, Arata said he will work part time for some Washington businesses, but he declined to specify his assignments or employers.

He put achieving full regional financing for Metro and the convention center as the city's highest priorities.