Potassium efflux from rat extensor digitorum longus muscle was measured by counting the decrease in K42 in muscle bathed in K42-free solution. The rate constant for K efflux was 0.27 hr.–1 at 37°C and 0.16 hr.–1 at 26°C. Insulin decreased the rate constant for K efflux to 0.22 hr.–1 at 37°C and to 0.09 hr.–1 at 26°C. The effect of insulin on efflux was quantitatively the same when glucose was removed from the bathing solution. From measurement of net change in intracellular K and from measurement of changes in the driving potential for K it is concluded that insulin probably also decreases K influx, but to a lesser degree. However, when half the chloride is removed from the bathing solution, efflux and influx are decreased equally. The data can be interpreted by the hypothesis that insulin increases the positive fixed charge within the muscle membrane.