The epileptogenic activity of several derivatives of beta-lactam was compared following their intracerebroventricular administration in rats. At a dose of 0.033 mumol/kg cefazolin was the most powerful epileptogenic compound among the drugs tested; dramatic seizure signs (nodding, clonic convulsions and sometimes escape responses) were observed repeatedly. It was approximately three times more potent than benzylpenicillin and other similar compounds, such as ceftriaxone, cefoperazone and cefamandole. No epileptogenic signs were observed with equimolar doses of cefotaxime, cefonicid and ceftizoxime. All these derivatives differ from the substitution at position 3 and at position 7 of 7-aminocephalosporanic acid. The more convulsant compounds (i.e. cefazolin and ceftezole) are both derivatives of tetrazol and show a marked similarity with pentylentetrazol. In addition, aztreonam, a compound having only the beta-lactam ring substituted with a heterocyclic ring at the 4 position, possessed convulsant properties like those of cefoperazone and cefamandole. This suggests that the beta-lactam ring is able to produce epileptogenic activity and it seems likely that substitutions at the 7-aminocephalosporanic or 6-aminopenicillanic acid may increase or reduce the epileptogenic properties of beta-lactam derivatives.