In various animal species (chicks, rats and rabbits) the intravenous injection of high doses of cefazolin, a cephalosporin not hitherto reported to produce epileptic seizures, was found to produce epileptiform electrocortrical changes similar to those evoked by intravenous benzylpenicillin. Similar phenomena as those reported after systemic injection were also observed after microinjection of cefazolin into the III cerebral ventricle. In fact, both in chicks and rats cefazolin given by the latter route produced wild running crisis, myoclonic jerks of the limbs and in some instances generalized clonic convulsions. Concomitantly, bilateral electrocortical high voltage bursts of spikes were observed, followed during the intercritic period by periodic bilateral or unilateral single spikes.