1. Isolated outer hair cells (OHCs) from the guinea-pig were whole-cell voltage clamped to study the influence of initial voltage on the voltage dependence of motility-related gating current or, equivalently, on the voltage dependence of membrane capacitance. 2. Prepulse delivery caused changes in the magnitude of motility-related gating currents, which are due predominantly to shifts in the voltage at peak capacitance (VpkCmm). Depolarization shifts VpkCm in the hyperpolarizing direction, and hyperpolarization does the opposite. The mean shift between -120 and +40 mV prepulse states with long-term holding potentials (> 2 min) at -80 mV was 14. 67 +/- 0.95 mV (n = 10; mean +/- s.e.m.). 3. The effect of initial membrane potential is sigmoidal, with a voltage dependence of 23 mV per e-fold change in VpkCm, and maximum slope within the physiological range of OHC resting potentials. This indicates that the cell is poised to respond maximally to changes in resting potential. 4. The kinetics of prepulse effects are slow compared with motility-related gating current kinetics. High-resolution measurement of membrane capacitance (Cm) using two voltage sinusoids indicates that shifts in VpkCm induce Cm changes with time courses fitted by two exponentials (tau0, 0.070 +/- 0.003 s; tau1, 1.28 +/- 0.07 s; A0, 1.54 +/- 0.13 pF; A1, 1.51 +/- 0.13 pF; means +/- s.e.m. ; n = 22; step from +50 to -80 mV). Recovery of prepulse effects exhibits a similar time course. 5. Prepulse effects are resistant to intracellular enzymatic digestion, to fast intracellular calcium buffers, and to intracellular pressure. Through modelling, we indicate how the effect may be explained by an intrinsic voltage-induced tension generated by the molecular motors residing in the lateral membrane.