The fast mechanical response is believed to be the cellular basis of the positive feedback mechanism required for the fine tuning process of the hearing organ. Using an externally applied electric field with and without membrane permeabilizing agents, we demonstrated that the fast mechanical response of the outer hair cell is membrane potential dependent. To clarify further how a change in the membrane potential causes mechanical displacement of the cell, we examined the role of the calcium ion in the fast response. The result indicated that the calcium ion is a modifier and not a messenger. We found stretch-activated potassium channels in the lateral wall, indicating that the lateral wall, as well as the stereocilia, can act as a mechanoreceptor.