The authors sought to determine whether different responsiveness to seizures induced by aminophylline existed between the genetically epilepsy-prone and normal rats. It was found that the seizure latency was consistently shorter in the genetically epilepsy-prone rats than in normal ones. A different pattern of response was observed in the progression to tonic seizures. In addition, seizures appeared to be more marked in genetically epilepsy-prone than in normal rats. A pretreatment with some quinolones (nalidixic acid, pipemidic acid, ciprofloxacin, norfloxacin, ofloxacin and enoxacin) significantly increased the convulsant properties of aminophylline. These studies demonstrated that the order of proconvulsant activity was ciprofloxacin greater than enoxacin greater than ofloxacin greater than norfloxacin greater than nalidixic acid greater than pipemidic acid. In addition, the present results showed that quinolones, having a fluorine atom showed the most marked proconvulsant activity.