Intracerebroventricular injection of penicillin G potassium (17.5, 35, 50, and 75μg) produced clonic convulsions, tonic flexor and extensor convulsions, and death due to respiratory embarassment in mice. The convulsion pattern was quite similar to that by peripherally administered picrotoxin, pentylenetetrazol (PTZ), or bemegride except strychnine which did not show marked tonic flexion. The tonic extension and death following penicillin increased in a dose-dependent manner. In order to clarify features of penicillin-induced convulsions, effect of various anticonvulsive drugs on penicillin-induced convulsions was compared with that on fits produced by PTZ or maximal electric shock (MES). Both the tonic extension and death induced by 50 μg of penicillin were prevented by the pretreatment with anticonvulsive drugs, phenytoin, phenobarbital, trimethadione, diazepam, or mephenesin, while chlorpromazine showed no anticonvulsive action. The penicillin-induced tonic extensor convulsion was, in general, more sensitive to anticonvulsive drugs than PTZ- or MES-induced convulsion. These results seem to suggest that penicillin-induced convulsion provides a useful tool for screening test of anticonvulsive actions of drugs.