The combined administration of enoxacin, a new quinolone, and fenbufen, a non-steroidal anti-inflammatory drug induced convulsions in mice, but convulsion did not occur when a single administration of each drug was given. Inhibition by enoxacin of [3H]muscimol binding to mouse and human brain membranes was remarkably facilitated by fenbufen. These inhibitions were much more prominent in the hippocampus and frontal cortex than in the cerebellum of human and mouse brain. Enoxacin and fenbufen each at a high concentration produced a moderate and slight inhibition of the [3H]muscimol binding, respectively, in the hippocampus and frontal cortex but not in the cerebellum. It would thus appear that the drug interaction between enoxacin and fenbufen on the gamma-aminobutyric acidA receptor in hippocampus and frontal cortex plays a role in inducing convulsions.