The effects of phorbol myristate acetate (PMA) and dexamethasone on protein kinase C (PK-C) activity and eicosanoid production were characterized in primary cultures of rat glial cells. PMA (1,000 ng/ml) treatment for 2 hr resulted in a maximal effect (a 4-fold increase in PGE, production). Longer exposure to PMA (up to 96 hr) resulted in attenuation of PGE, production. Down-regulation of PK-C activity was assessed in glial cell homogenates under these conditions. Although a 70% inhibition of PK-C activity was measured upon staurosporine treatment, PGE, production was not affected both under basal conditions and following PMA activation. The production of thromboxane B, did not change following exposure to PMA. Pretreatment of the cultures with dexamethasone markedly inhibited the PMA-stimulated production of PGE, but had only a moderate (approximately 26%) inhibitory effect on PGE, production under basal conditions. Dexamethasone had no effect on basal or PMA-stimulated PK-C activity. Forskolin, which activates adenylate cyclase, did not affect PGE, production. These data may suggest that activation of PGE, production by PMA in glial cells is not unequivocally mediated by PK-C activation. The inhibitory effect of dexamethasone on the PMA-stimulated synthesis of PGE, supports previous findings that glucocorticoids are more effective in inhibiting stimulated rather than basal PGE, production.