Glial fibrillary acidic protein (GFAP) is expressed upon cAMP-mediated induction of differentiation of glial progenitor cells into type II astrocytes. The protein is regulated by hormones, growth factors and cytokines but the signal transduction pathways involved in the regulation of GFAP expression are largely unknown. Specific protein kinase inhibitors were used to study their effect on the expression of GFAP in rat C6 glioma cells. Herbimycin A, a selective protein tyrosine kinase inhibitor, reduced GFAP mRNA and protein expression upon cAMP analog or β-adrenergic receptor-mediated induction of differentiation. The latter inhibitor attenuated the elevation of cAMP by adenylate cyclase and abolished the activity of phosphatidylinositol 3-kinase (PI 3-K). These data indicate that GFAP expression is regulated by protein tyrosine phosphorylations, modulating the cAMP concentration and PI 3-K activity in C6 glioma cells.