Forskolin, a diterpene activator of adenylate cyclase, stimulates the production of cyclic adenosine monophosphate (cAMP) in a wide variety of cell types. In C6 glioma, used in this study, the anticonvulsant agent valproic acid (VPA) inhibited forskolin-stimulated cAMP accumulation in intact cells in a concentration-dependent manner. Kinetic studies indicated this valproate effect not to be mediated by direct inhibition of adenylate cyclase activity. The valproate-induced inhibition of cAMP accumulation was partially reversed by the phosphodiesterase (PDE) inhibitor isobutylmethyl xanthine (IBMX). Degradation of cAMP over time was more rapid in valproate-treated cells than in controls, and this effect was also reversed by IBMX. In synchronised C6 glioma, phosphodiesterase type IV (PDE4A1) expression was selectively upregulated during the G1 phase, in tandem with temporal biphasic peaks of cAMP. However, the expression of PDE4 isoforms was not affected by a 48-h exposure to valproate. These findings suggest inhibition of forskolin-stimulated cAMP levels in C6 glioma by valproate to be mediated by increased activation of PDE in the G1 phase. Since the degree of cell cycle arrest induced by valproate is intimately associated with its teratogenic potency, it appears that PDE-mediated inhibition of cAMP may contribute to the molecular mechanisms of valproate-induced teratogenicity.