The dependence of the membrane potential oscillation on extracellular sodium concentration was studied in the frog sartorius muscle. (1) When Na concentration of the Ringer solution was decreased to zero, the membrane potential oscillation cesed and the membrane tended to repolarize. This inhibitory effect was reversible. (2) Tetrodotoxin, 10(-7) g/ml, inhibited the development of membrane potential oscillation or stopped within 2--4 min the membrane potential oscillation already developed, the membrane then became partly repolarized maximum to -72mV. The inhibitory effect of tetrodotoxin was also reversible. (3) When the Na concentration of the Ringer solution was reduced to 60 mM the latency time of the membrane potential oscillation did not change. By further decreasing the Na concentration the latency time increased in a hyperbolic function. In case of 10 mM Na the latency period was 7 times longer than the control. (4) The depolarizing effect of veratrine was linearly related to the logarithm of Na concentration in a range from 10 to 120 mM Na. (5). By decreasing the [Na]c from 120 to 10 mM the frequency of oscillation decreased and the amplitude increased linearly as a function of the logarithm of Na concentration.