Intracellular recordings were made from the low frequency region (third turn) of the guinea pig cochlea. Response characteristics are compared to gross potentials obtained from the organ of Corti fluid space. Inner hair cells (IHCs) possess relatively low (median, -32 mV) initial membrane potentials, whereas that of outer hair cells (OHCs) is higher (median, -53.5 mV). In response to tone burst stimuli, both cell types produce a combination of AC and DC responses. The latter are depolarizing for IHCs but may be of either polarity for OHCs. In terms of their AC responses, IHCs are about 12 dB more sensitive than OHCs. At low sound levels these cells are more linear than high frequency hair cells (Russell, I. J., and P. M. Sellick (1978) J. Physiol. (Lond.) 284: 261-290), judging from the relation between AC and DC response components. At high sound levels pronounced response saturation is seen. The overall tuning properties of the two hair cell types are rather similar, even though IHCs exhibit low frequency velocity dependence, whereas OHCs are displacement sensitive and the cell membrane time constant is larger for IHCs. In order to fit IHC experimental data it is necessary to assume the presence of an underdamped complex pole above the best frequency. The electrical behavior of the OHC does not disqualify it as a conveyor of auditory information to the central nervous system, even though its primary function may be that of a mechanical effector (evidence summarized by Dallos, P. (1985) in Contemporary Sensory Neurobiology, Alan R. Liss, Inc., New York, pp. 207-230).