Organotypic explant cultures were prepared from the cochleas of 1 to 3 day post-natal mice and maintained in vitro for up to 5 days. The hair cells retain morphological integrity for the duration of the culture period although they exhibit embryological features such as a kinocilium and additional microvilli on their apical surfaces. The resting membrane potentials of mouse inner hair cells (IHCs) in vitro are similar to those of guinea-pig IHCs in vivo but the membrane potentials of outer hair cells (OHCs) in the mouse cochlea in vitro are less polarized than the resting membrane potentials of OHCs in the basal turn of the guinea-pig cochlea in vivo. The voltage responses of IHCs and OHCs to sinusoidal displacements of their stereocilia are similar to each other in waveform and dynamic range, although the responses of IHCs are larger than those of OHCs. The relationship between transducer conductance and stereocilia displacement in IHCs and OHCs is non-linear and largely accounts for the depolarizing asymmetry of the voltage response. The receptor potentials of IHCs and OHCs reverse close to 0 mV indicating that the transducer conductance is non-selective for cations. The voltage responses of IHCs and OHCs to intracellular current injection rectify when the membrane potentials are more depolarized than about -30 mV. This rectification is most pronounced in OHCs. OHCs also exhibit a time-dependent, voltage-sensitive conductance although they do not behave as electrical resonators.