Virginia Gov. Gerald L. Baliles promoted Howard M. Cullum today to commissioner of the Department of Mental Health and Mental Retardation. Cullum has been acting head of the department since December when commissioner Joseph J. Bevilacqua left to take a similar job in South Carolina.

Baliles, who with today's announcement has chosen or reappointed six cabinet secretaries and nearly all department heads, said he would make the remaining appointments within 30 days. He also must fill the new cabinet position of secretary of natural resources, created by the legislature at his request.

Among holdovers from the administration of former governor Charles S. Robb who have not been reappointed are Superintendent of Public Instruction S. John Davis, Agriculture Commissioner S. Mason Carbaugh and Health Commissioner James B. Kenley.

Cullum, 42, a former human resources administrator in Arlington and Fairfax counties, will be paid $74,818 a year.

Cullum, who promised that he would operate a "client-focused administration," holds degrees in government from Miami University in Ohio and a master's degree in public administration from American University.

He worked in Arlington from 1971 to 1973 and in Fairfax from 1974 to 1977 before going in 1977 to Virginia Beach, where he was executive director of its community services board. He joined the state agency in July 1983 as deputy commissioner.

Baliles said he has directed Cullum to tackle four questions immediately: the staffing needs of various mental health programs, the lack or loss of licensing compliance of state institutions, the potential for "privatization" in the field, and -- in conjunction with the secretaries for economic development and human resources -- a plan for serving the needs of the homeless.

Baliles said he has asked Cullum to "resolve the question of where to draw the line between the institutionalization and deinstitutionalization approaches" for mental patients, adding that he wanted "the emphasis to be on treatment services for individuals in need rather than on the battle between the bureaucracies at the state and local levels."

The legislature, at Baliles' request, authorized four pilot programs designed to test several possible ways of providing those services. These included a highly supervised inpatient treatment using vacant hospital buildings, halfway houses in local communities and support for families of patients, such as respite care, which provides relief for family members who are caring for patients in their homes.

A budgetary matter that needs attention, Baliles said, is whether more money should be shifted from hospitals, whose populations have decreased by about 50 percent in the last 10 years, but which still command nearly 60 percent of the mental health budget.