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The time course of relaxation of the electric current following steps in the applied potential across lipid bilayer membranes has been measured. The membranes were made cation-selective by the addition of nonactin. To permit the measurement of very short time constants a voltage clamp device was developed in order to reduce the charging period to less than 1 musec, regardless of the magnitude of the series resistances in the external solutions. It was possible by this method to establish the presence of two electric processes, which were found to behave differently with respect to temperature, applied potential and external solution conditions. The rapid process (tau approximately 10 musec) was interpreted in terms of the electric parameters of the polar part of the membrane according to the theory developed by Hägglund and Sandblom (T.I.T.J. Life Sic. 2: 107, 1972). The second process (tau approximately 100 musec) showed a behavior consistent with the model of Stark, Ketterer, Benz and Läuger (Biophys. J. 11:981, 1971), which considers the different rate constants involved in the net transfer of carriermediated ion transport across bilayer membranes.