The action of met-enkephalin on the growth of astrocytes in mixed-glial cultures was examined. Primary, mixed-glial cultures were isolated from 1 day-old mouse cerebral hemispheres and continuously treated with either basal growth media (controls), 1 microM met-enkephalin, 1 microM met-enkephalin plus the opioid antagonist naloxone (3 microM), or naloxone alone (3 microM). Absolute numbers of neural cells were counted in unstained preparations, while combined [3H]-thymidine autoradiography and glial fibrillary acid protein (GFAP) immunocytochemistry was performed to identify specific changes in astrocytes. When compared to control and naloxone treated cultures, met-enkephalin caused a significant decrease in both total cell numbers, and in [3H]-thymidine incorporation by GFAP-positive cells with flat morphology. These results indicate that met-enkephalin suppresses astrocyte growth in culture.