Paraoxon, the active metabolite of parathion, is an acetylcholinesterases (AChE) inhibitor that kills cultured cerebellar granule cell neurons via an apoptotic mechanism. Protein kinase C is an enzyme with diverse functions but its role in paraoxon-induced cell death is unknown. We show that a neurotoxic concentration of paraoxon increases PKC phosphorylation. We tested whether PKC is involved in paraoxon-induced neuronal cell death by using the PKC activator, phorbol 12-myristate 13-acetate (TPA). TPA increases PKC activity and enhances the neurotoxic effect of paraoxon by 28%. In sharp contrast, addition of the PKC inhibitor Ro-31-8220 protects more than 30% neurons that would otherwise die from paraoxon-induced neuronal cell death in either a pretreatment or post-treatment paradigm and markedly reduces phospho-PKC pan levels. We also show that the pretreatment of Ro-31-8220 blocks paraoxon-induced caspase-3 activity completely. These results suggest that activation of protein kinase C is required for paraoxon neurotoxicity.