Abstract: Streptomycin showed negative inotropic effects on the isolated hearts (toad, rabbit, and guinea-pig). The contractions of isolated rabbit ileum and rat uterus were also in- hibited by streptomycin. In the experiments on perfused isolated rabbit ear, streptomycin had no effect on the outflow of perfusing fluid. The contractive responses of rabbit aorta strips to adrenaline was neither prevented nor abolished by streptomycin. Calcium ions antagonized the depressing effects of streptomycin on the isolated rat uterus, but did not influence the inhibitory effects of streptomycin on isolated hearts (toad, rabbit, and guinea-pig) and rabbit ileum. Our experiments showed that calcium ions only partly antagonized the pharmaco- logical action of streptomycin on certain isolated organs.