Electrical rectification was demonstrated in whole sartorius muscle and sciatic nerve of Rana pipiens and also in the single giant nerve fiber of the northern squid, Ommastrephes illecibrossus. It is probably a property of the plasma membrane. Rectification decreases reversibly under the influence of increased concentrations of the potassium ion and with chloroform, veratrine sulfate and isoamyl carbamate. No effect was found with lack of calcium, excess calcium, or barium chloride. Decrease in rectification is invariably accompanied by simultaneous decrease in resting potential. A proposed explanation of the mechanism of rectification is discussed. Rectification in a living membrane, viz. a change in resistance with change in direction of current flow, may possibly be explained in terms of a change in the concentration of potassium ions in the membrane.