The present experiments were designed to evaluate the effectiveness of forskolin on cAMP production, growth and morphology on cell cultures of glia, endothelium and smooth muscle derived from brain microvessels. Forskolin significantly increased formation of cAMP and decreased incorporation of thymidine in all three cell types. The thymidine incorporation was reduced dose-dependently with maximal growth inhibition at 100 microM forskolin. A 1 hr preincubation with forskolin abolished thymidine incorporation by cells grown in fetal calf serum (FCS)-containing media over the following 24 hr. In cerebromicrovascular endothelium and smooth muscle, forskolin caused drastic and immediate changes of cell morphology and F-actin composition that were reversible. In glial cells, morphological changes were visible only after exposure to forskolin for more than 24 hr. These changes were accompanied by increased staining with antibodies against glial fibrillary acidic protein (GFAP). These findings support the contention of cAMP involvement in growth regulation of these cells and indicate that forskolin might be used as a tool to induce growth arrest and possible differentiation in cell cultures from mammalian brain.