Outer hair cells (OHC) from the organ of Corti are capable of fast voltage-induced length changes (Santos-Sacchi and Dilger, 1988), suggesting that an associated voltage sensor should reside in the OHC plasma membrane. Voltage-dependent mechanical responses and nonlinear charge movement of isolated OHCs from the guinea pig were analyzed using the whole-cell voltage-clamp technique. Ionic currents in the cells were blocked. Nonlinear voltage-dependent charge movement or, correspondingly, voltage-dependent capacitance was measured with step or AC analysis. OHC movements were measured either from video or using a differential photodiode technique. Maximum charge movements up to 2.5 pC were measured in OHCs from the low-frequency region of the cochlea. Both AC and step analyses indicated a peak nonlinear capacitance of 16- 17 pF. The voltage dependence was fit to a Boltzmann relation with the step analysis indicating a maximum nonlinear capacitance at -23 mV step potential from a holding potential of about -120 mV, and AC analysis indicating a maximum at a holding potential near -40 mV. AC analysis probably provides a more accurate evaluation of voltage dependence. Measures of OHC motility magnitude versus voltage follow the nonlinear capacitance-voltage function obtained from AC measures. Treatment of the cells with gadolinium ions (0.5-1 mM) blocked OHC motility. This treatment also produced a shift of the nonlinear capacitance function along the voltage axis in the depolarizing direction, which can be explained by membrane surface charge screening. However, maximum capacitance was reduced as well and may correspond to the reduction or abolition of OHC motility in response to gadolinium treatment. Gadolinium effects were reversible. Nonlinear capacitance is not a function of membrane deformation due to length changes, since removal of OHC cytosol via the patch pipette abolished longitudinal movement but did not reduce nonlinear charge movement. It is interesting to note that the nonlinear capacitance will dynamically influence the time constant of the OHC during acoustically evoked receptor potential generation.