Convulsant activity of pazufloxacin mesilate (PZFX mesilate), a new quinolone antibacterial agent for intravenous use, in combination with nonsteroidal anti-inflammatory drug (NSAID) was investigated in mice after intravenous or intracerebroventricular administration. Following results were obtained. 1. In combination with 4-biphenylacetic acid (BPAA) at an oral dose of 100 mg/kg, PZFX mesilate did not induce any convulsions at intravenous doses up to 200 mg/kg. Reference quinolones induced convulsions at the following intravenous doses: Enoxacin (ENX), 3.13 mg/kg or more; norfloxacin (NFLX) and lomefloxacin (LFLX), 6.25 mg/kg or more; ciprofloxacin (CPFX), 50 mg/kg or more; sparfloxacin (SPFX) and temafloxacin (TMFX), 100 mg/kg or more; fleroxacin (FLRX), 200 mg/kg. 2. PZFX mesilate at an intravenous dose of 50 mg/kg did not induce convulsions in mice after oral administration of any of 14 kinds of NSAIDs. It induced convulsions at 200 mg/kg in combination with aspirin at an oral dose of 600 mg/kg, while it did not with the other 13 kinds of NSAIDs. 3. Convulsion-inducing dose of PZFX mesilate after intracerebroventricular administration was 100 mg/body, which was higher than those of reference quinolones (NFLX, CPFX, ENX, LFLX, TMFX, levofloxacin, ofloxacin, FLRX and SPFX) and beta-lactam antibiotics (penicillin G, cefazoline, imipenem/cilastatin and panipenem/betamipron). In addition, concurrent dosing of BPAA (1 microgram/body) did not reduce the convulsion-inducing dose of PZFX mesilate. These results suggest that PZFX mesilate has remarkably weak convulsant activity.