Cephalosporins have been well known to have potent convulsant activity. We studied the mechanism of the convulsions induced by cephalosporins. Cefazolin, cephaloridine, cefpirome and cefmetazole inhibited the receptor binding of γ-aminobutyric acid (GABA), an inhibitory transmitter in the mammalian central nervous system. As fluoroquinolones also inhibited GABA receptor binding and this inhibition was enhanced in the presence of non-steroidal anti-inflammatory drugs (NSAIDs), we studied the effect of cephalosporins on GABA receptor binding in the presence of NSAIDs. The inhibitory activity of cephalosporins was not enhanced in the presence of NSAIDs. These results suggested that cephalosporins might induce convulsions through the inhibition of GABA receptor binding, and that concurrent administration of cephalosporins and NSAIDs might not enhance the convulsant activity of cephalosporins.