Carolyn L. Smith, director of the D. C. Department of Finance and Revenue, said last night that she has resigned the post, effective Nov. 12, to return to private business.

"I'm quitting to further my career," said Smith, 39, whose department has played a major role in forecasting revenues as the city struggled through a financial crisis in the early years of Mayor Marion Barry's administration. She has served in the post since November 1979.

The city is facing new economic constraints because of the slumping economy and declining revenues, and Smith's department is preparing to reduce its revenue forecast for the coming year.

Smith's department, in addition to predicting the city's revenues based on expected tax collections, also advises the mayor on tax policy, including increases in sales and property taxes.

Smith, who has served as the city's chief tax official since November, 1979, is the first top Barry appointee to confirm -- in an interview with a reporter last night -- that she will leave the government since Barry won the Sept. 14 Democratic primary nomination. Three days after his nomination, Barry called for the pro forma resignations of 48 department heads and cabinet officers to give him a free hand in reshaping the city government.

James Buford, director of the Department of Human Resources, reportedly told the mayor in September that he plans to leave his job in December, but the city and Buford have declined to confirm that he will leave.

Smith joined the city during Walter Washington's administration and served two years as the District's treasurer before being named to her current post.

Although city council members have criticized the Barry administration as reluctant to release financial data, Smith has won respect for her department's work in forecasting revenues for the city during a period of economic uncertainty.

Smith, a certified public accountant, said she told Barry last summer that she would return to her former job as an auditor and consultant with the Washington office of Coopers & Lybrand, an international accounting firm.

There, she said last night, her salary will be "a lot better than the $57,000" she now makes.

Jeffrey L. Humber, who recently was named Smith's deputy director, will become acting director of the office, Smith said.