Summary. The effect of a direct-current electric field on films of brain cerebroside and cholesterol and mixtures with brain phosphatidylethanolamine (PE) using infrared spectral techniques was studied. The intensities of the spectral bands assigned to the vibration of the phosphate and the -(CH2),-grou p of the fatty acid chain of the phospholipid increased to a maximum as the applied voltage increased. The changes were similar to those previously observed with films of the phospholipid, but higher voltages were required to reach the maximum than with the phospholipid films. The spectral changes are related to conformational changes occurring in the phospholipid component of the films containing cerebroside and cholesterol. The infrared spectra of the mixtures indicate that there is hydrogen bonding between the phosphatidylethanolamine and cholesterol in the films and that the phospholipid exists in the nonpolar form in the films. As found previously with films of brain cephalin and lecithin, the electric field strength at which the maximum intensities of spectral bands are observed varies inversely with the thickness.During the excitation of the neuron, changes occur in the permeability of the membrane to ions. These changes may be associated with alterations in the molecular configuration of the membranal constituents some of which are lipids. The behavior of films of lipids may provide information concerning the changes that occur in these substances in the neuronal membrane during excitation. Films are used because they can be considered as models for the lipid structures of the membrane.Previous observations on the effect of direct-current electric fields on films of the brain phospholipids, cephalin and lecithin, have shown that the intensities of certain infrared absorption bands were altered [7]. Since the phospholipids exist in the membrane in combination with cerebroside (CEB) and cholesterol (CHOL), the effect of the interaction between these lipids could be determined by performing similar spectral measurements with mixtures of the lipids and CHOL. A mixture of PE-CEB-CHOL with the molar ratio of 1:0.5:1 was selected since this represented the