Comparing with other pyridonecarboxylic acids (PCAs), the neurotoxicity of (+/-)-7-(3-amino-1-pyrrolidinyl)-6-fluoro-1-(2,4-difluorophenyl)-1,4- dihydro-4-oxo-1,8-naphthyridine-3-carboxylic acid p-toluenesulfonate hydrate (T-3262), which is a new PCA, was investigated in mice in a combination with fenbufen (FBF). T-3262, ofloxacin (OFLX) and nalidixic acid (NA) did not produce convulsion, but enoxacin (ENX) and norfloxacin (NFLX) produced it after an oral administration with FBF. The intracerebral injection of drugs alone to mice revealed that both FBF and 4-biphenylacetic acid (BPAA), which is principally responsible for FBF's antiinflammatory action, scarcely had convulsant activity. While all the PCAs had convulsant activity and the order of potency was as follows; NFLX greater than ENX greater than OFLX greater than penicillin G potassium greater than the free base of T-3262 (T-3262 base) greater than or equal to NA. When orally pretreated with BPAA, the convulsive threshold was scarcely lowered for T-3262 base and OFLX, but for ENX and NFLX it was lowered to about 1/300 and 1/100 of the respective activity. As the result, convulsant activities of ENX and NFLX were greatly potentiated, and their potencies became almost equal. The adverse drug interactions between T-3262 and FBF were scarcely observed in mice.