The effect of kanamycin and gentamicin on frog n.sciatic-m.gastrocnemian preparatus was analyzed. Both substances decreased the muscular contraction in both after direct electrical stimulation of the muscle and electrical nerve stimulation, in a dose-dependent manner. Both substances were used at a concentration near the therapeutical concentration in human. The effect of kanamycin was more important than the effect of gentamicin. These data obtained in presented experimental condition are in accord with clinical observation but are not in accord with the possibility that aminoglycoside antibiotics block presynaptic Ca2+ channels.