Isolated outer hair cells from the organ of Corti show elongation and contraction in response to an externally applied ac electric field as well as to a direct current injection into these cells. This is thought to be the basis of the positive feedback mechanism for fine tuning of the mammalian hearing organ. To test whether the mechanical response depends on the intracellular electric field or on the membrane potential, we used digitonin to shunt the membrane resistance. We observed that the application of digitonin abolished the cellular response of the outer hair cells to an ac external electric field (5-30 Hz). Coinciding with the abolition of the cellular response, the nuclear matrix started to oscillate synchronous to the external field, indicating an appreciable increase of the intracellular electric field. If the intracellular electric field was the regulating factor of the motile response, the initiation of the movement of the nuclear matrix would have been accompanied by an enhancement of the cellular movement. Our observation is therefore consistent with the interpretation that the (local) membrane potential, and not the intracellular electric field, regulates the hair cell movement.