The present study was undertaken to investigate the effects of kanamycin (KM), bekanamycin (AKM) and ribostamycin (RSM), aminoglycoside antibiotics, onthe contractile response of isolated guinea pig was deferens. The contractile response of isolated was deferens induced by electrical stimulation with rectangular pulses (50 volt, 5 Hz) of 0.5 msec duration for a period of 5 sec was abolished by guanethidine and tetrodotoxin, but was not affected by hexamethonium. Therefore, the intramural nerves supplying the was deferens are undoubtly the postganglionic sympathetic nerves, and the above-mentioned ontractile response may be due to a stimulus effect of the electrical current on the intramural nerves of the was deferens (electrical nerve stimulation). Further, the contractile response of isolated was deferens induced by electrical stimulation with rectangular pulses (50 volt, 5 Hz) of 50 msec duration for a period of 5 sec in the presence of tetrodotoxin may be due to a stimulus effect of the electrical current on the muscle of the was deferens (electrical muscle stimulation). It has been proposed, moreov r, that noradrenaline and adenosine triphosphate are simultaneously released from sympathetic nerves in the was deferens, and act as co-transmitters, while the isolated was deferens evoked a contractile response by exogenously added noradrenaline as well as exogenously added adenosine triphosphate. KM (1 ×10^-5 g/ml-1 ×10^-3 g/ml), AKM (5 ×10^-6 g/ml-1 ×10^-3 g/ml) and RSM (5 × 10^-5 g/ml-1 ×10^-3 g/ml) reduced the amplitude of the contractile response induced by electrical nerve stimulation in a concentration-dependent manner. KM, AKM and RSM, each at a concentration of 5 ×10^-4 g/ml, reduced, withmuch approximation, the amplitudes of contractile responses induced by the three treatments, that is, electrical muscle stimulation, exogenously added noradrenaline and exogenously added adenosine triphosphate. However, each of these antibiotics at the concentration mentioned was exceedingly potent in reducing the effect on the amplitude of the contractile response induced by electrical nerve stimulation. All these findings may bring about a conclusion that KM, AKM and RSM affect both intramural sympathetic nerves and muscle of the was deferens. It also seems possible that the effects of these antibiotics are greater on the former than they are on the latter.