Summary of Work: The outer hair cell is a critical factor for fine tuning and for the wide dynamic range of the mammalian ear. This cell has an unusual motility which is dependent on the membrane potential and not on ATP or calcium ion. To clarify the motile mechanism, both motile and passive properties of the cell have been studied. Deformations of the cell due to pressure are explained by a simple elastic membrane model. Our recent studies on the axial stiffness show that the cell membrane is indeed not too far from a mechanical isotropy. The motility is associated with the membrane capacitance of the cell that is dependent on membrane tension as well as membrane potential. The source of this capacitance is a transfer of a charge associated with the motor across the membrane. These observations reveal that this motility is based on a membrane motor which directly utilizes electric energy. The motor and the mechanical characteristics of the outer hair cell are incorporated into a theoretical model, predicting force generation of 0.1 nN/mV for a single cell. Our recent data indeed support this prediction. To clarify the structure function relationship, we examined the effect of diamide, a sulfhydryl oxidizing reagent. This chemical reduced the axial stiffness and the force production of the cell which confirmed the prediction of the model. Further efforts to clarify this motility and identify the role of the outer hair cell in hearing are planned.