We have used a high-resolution motion analysis system to reinvestigate shape changes in isolated guinea pig cochlear outer hair cells (OHCs) evoked by low-frequency (2-3 Hz) external electric stimulation. This phenomenon of electromotility is presumed to result from voltage-dependent structural changes in the lateral plasma membrane of the OHC. In addition to well-known longitudinal movements, OHCs were found to display bending movements when the alternating external electric field gradients were oriented perpendicular to the cylindrical cell body. The peak-to-peak amplitude of the bending movement was found to be as large as 0.7 microm. The specific sulfhydryl reagents, p-chloromercuriphenylsulfonic acid and p-hydroxymercuriphenylsulfonic acid, that suppress electrically evoked longitudinal OHCs movements, also inhibit the bending movements, indicating that these two movements share the same underlying mechanism. The OHC bending is likely to result from an electrical charge separation that produces depolarization of the lateral plasma membrane on one side of the cell and hyperpolarization on the other side. In the cochlea, OHC bending could produce radial distortions in the sensory epithelium and influence the micromechanics of the organ of Corti.