The Na/Ca exchange system was investigated by contractile response to alteration of the extracellular sodium concentration in tonic skeletal muscle fibers of the frog. Contractures were evoked when the extracellular sodium was reduced or withdrawn in the normal solution. This effect was not associated with membrane depolarizations. In the presence of d-tubocurarine, the amplitude and time course of sodium withdrawal contractures were modified, except when sodium was replaced by TEA. When external calcium was omitted from the solution, the tension of sodium withdrawal contracture was greatly reduced. This effect was reversible. These results suggest that Na/Ca exchange is present in the membrane of tonic skeletal muscle fibers of the frog. This conclusion is further supported by the effect of veratridine and strophantidin, which increase the tension of the low-sodium contractures.