As Carmen E. Turner completed her seven years as Metro general manager yesterday, the chairman of the Metro board said transit officials probably will begin interviewing potential successors next month.

The chairman, Mary Margaret Whipple, of Arlington, said a Boston executive search firm is expected to give the board a list of candidates within two weeks. The board then will decide whom to interview in January, with a selection probably by spring.

Whipple said the board is moving "with all deliberate speed" and sought to assure the riders who take a million rail and bus trips each weekday that the system will be run safely and efficiently in the interim.

But the pace of the selection process has bothered other board members at a time when Metro is struggling. The area transit agency announced Thursday that up to 335 jobs will be cut and other spending reduced to offset a $12.5 million deficit in the current budget year.

"We have a financial situation very different than it was seven years ago," said Joseph Alexander, a board member from Fairfax. "I'm concerned about the slowness of the process."

Because the Metro job is one of the most prestigious in the transit industry, the list of candidates is long and includes some of the top names in the field, Whipple said. A tight lid has been kept on who is being considered.

In praising Turner this week, board members Frank Smith Jr., of the District, and Kate K. Hanley, of Fairfax, said she was a role model to women and black people. The board has not specified it wants another minority, but one frequently mentioned possibility is Shirley A. DeLibero, a black woman who has previous ties to Metro.

DeLibero, recently named the general manager of New Jersey Transit, was Metro's bus manager from 1982 to 1986. There are other top black managers in the Seattle and Detroit systems.

The list includes applicants from within Metro and may include local officials who have been mentioned, among them retiring Fairfax County Executive J. Hamilton Lambert and former Montgomery transportation chief Robert S. McGarry.

However, some board members said the board probably will choose someone who has a proven record as a manager in the transit field or similar enterprise.

Turner, 60, announced in October that she would leave Metro to become the chief operating officer of the Smithsonian Institution, which, like Metro, is experiencing financial difficulties.

Turner, a Washington native who spent 26 years in the federal government before joining Metro in 1977, will be a hard act to follow, as was noted in many tributes this week.

"She had the ability to communicate with a varied number and types of people about what Metro was all about and convinced them Metro was well run and worth the contribution," said Rep. Steny H. Hoyer. The Maryland Democrat was instrumental, with Turner, in getting Congress this fall to authorize $1.3 billion toward completion of nearly all unfinished parts of the planned 103-mile system.

Turner's low point was the failure to open the first Green Line stations in Northwest Washington on time this month. Turner fired the original contractor, which contended that Metro mismanagement effectively has delayed the opening until May 1991. The two sides still are tied up in expensive litigation.

Until a successor is picked, the board has named William A. Boleyn, Metro's 68-year-old finance chief, as interim general manager.

Boleyn, a 16-year veteran of Metro, was considered for general manager in 1983 but lost out to Turner.

He said in an interview that his immediate priority will be to tackle the agency's fiscal problems, starting with the $12.5 million deficit.

The budget proposal for the year beginning July 1, which will be submitted to the board next month, will contain more of the same: higher costs and lower revenue. The growth in ridership is down, the region's economy is slumping and the amount in subsidies required by local governments is increasing sharply. Yearly fare increases are likely.

"We think we can put the necessary restraints in place without impacting service," Boleyn said.