Montgomery County Executive Charles W. Gilchrist made the first of what is expected to be a series of high-level personnel changes this week by telling two department heads who are longtime county employes they will be replaced.

Gilchrist, a Democrat who replaced a Republican as county executive 10 weeks ago, is expected to notify more department heads on Tuesday of plans to replace them. The new executive has the power to appoint as many as 21 department heads and three special assistants, although it is unclear how many he intends to replace.

The two department heads told of their dismissal Thursday are Fran Abrams, director of the Department of Environmental Protection, and Hameed Naz, head of the Health Systems Planning Agency.

Despite weeks of rumors that environmental protection and health systems planning were among the agencies scheduled for new leadership, Abrams and Naz said yesterday they were surprised.

Naz said Gilchrist explained that he "wanted to put in his own cabinet" in order to have "the direction" he wants in his government.

Abrams, a specialist in architecture and urban planning, went to work for the county 10 years ago, taking over environmental protection 3 years ago as one of five women department heads named by Gilchrist's predecessor, James P. Gleason. The agency, traditionally regarded as an unwieldy bureaucracy, is in charge of the local government's licensing and regulatory functions.

Naz, an economist, worked for the county planning board and in the county's administrative office before being named head of the new Health Svstems Planning Agency in October 1976. Gilchrist must choose Naz's successor from a list submitted by the agency's board.

Gilchrist made two key appointments in his first two weeks in office, naming Robert Wilson as chief administrative officer -- chief of the county bureaucracy -- and Paul McGuckian as county attorney. He has also installed Blair Lee IV, son of former Acting Gov. Blain Lee III, as the county's lobbyist in Annapolis, and fired Police Chief Robert J. diGrazia.