A method was developed to standardize the induction of seizure activity during enflurane anaesthesia in cats. The effects of nitrous oxide on the epileptogenic property of enflurane were investigated. A potent anticonvulsant action of nitrous oxide was noted, but partial tolerance to this effect rapidly developed. Some evidence of withdrawal hyperexcitability was also detected. The changes induced by nitrous oxide, despite existing enflurane anaesthesia, suggest that these two anaesthetic agents have different sites or mechanisms of actions.