After the intracisternal injection of three protease inhibitors which prevent the degradation of methionine enkephalin (amastatin, Des-Pro2-bradykinin, and phosphoramidon) and a mixture of these protease inhibitors, we investigated the effect on convulsive seizures in the seizure-susceptible El mouse. We also measured the cerebral methionine enkephalin content by high-performance liquid chromatography coupled with radioimmunoassay. Protease inhibitors significantly decreased both the incidence of seizures and the seizure score in El mice in a dose-dependent manner. This anticonvulsant effect was reversed by naloxone (2 mg/kg, sc). The cerebral methionine enkephalin content increased significantly after the administration of protease inhibitors in comparison with saline injection. These findings suggest that it was not protease inhibitors but instead increase of endogenous methionine enkephalin that reduced the incidence of seizures and the seizure score in El mice. Together with our previous data, the present findings support our hypothesis that a deficit in anticonvulsant endogenous methionine enkephalin is involved in the pathogenesis of seizures in the El mouse.