General pharmacological effects of T-3761, a new oral quinolone antibacterial agent, on the central nervous system were investigated in laboratory animals. The results obtained are summarized as follows. 1. T-3761 exerted no significant effects on spontaneous motor activity, motor coordination, pentobarbital-induced hypnosis, electroshock-, pentetrazole- or strychnine-induced convulsion, acetic acid-induced writhing responses, reserpine-induced hypothermia and ptosis in mice at oral doses of 100, 300 and 1,000 mg/kg. The same oral doses of T-3761 exerted no significant effects on body temperature and passive avoidance response in rats. 2. T-3761 had no effects on EEG in cats and spinal reflex in rats at intravenous doses of 10, 30 and 100 mg/kg. 3. Convulsions were not observed in mice after any oral combinations of T-3761 at a dose of 200 or 1,000 mg/kg with 14 different nonsteroidal anti-inflammatory drugs (NSAIDs) including fenbufen. 4. An oral combination of T-3761 even at a higher doses of 3,000 mg/kg with 4-biphenylacetic acid (BPAA) which is a principally active metabolite of fenbufen also did not induce convulsions in mice. 5. T-3761 did not inhibit GABA receptor binding in rat brain synaptic membranes at 10(-4) M in either the absence or presence of BPAA. These results suggest that T-3761 is an antibacterial agent which would be unlikely to produce any side effects on the central nervous system and to produce convulsion when combined with NSAIDs in clinical use.