Intramuscular injection of diazepam into rats in doses of 0.01 and 2 mg/kg 25–30 min after application of penicillin to the cerebral cortex was shown to lead to a disturbance of the periodic occurrence of epileptic fits (EF), to a change in their character, and to the appearance of periods of increased frequency of epileptiform discharges (ED). Injection of diazepam in a dose of 2 mg/kg 20 min before application of penicillin led to shortening of the latent period of ED in the epileptic focus and to an increase in their frequency in the period before the fit compared with the control without diazepam. EF appeared irregularly, their number was sharply reduced, and their duration increased. Injection of diazepam led to disappearance of the animal''s motor response during ED and EF. Diazepam (2 mg/kg) in experiments in vivo did not affect Na,K-ATPase activity of unpurified cerebral cortical synaptosomes, but after creation of an epileptic focus it led to an increase in Na,K-ATPase activity in the primary focus and secondary foci dependent on it. It is suggested that the anticonvulsant action of diazepam may be based on its activating effect on Na,K-ATPase in neuron membranes in the epileptic focus.