Intracellular receptor potentials were recorded from inner hair cells in the basal turn of the guinea pig cochlea in response to low frequency tones. These were compared with the cochlear microphonic (CM) recorded from the scala tympani and sound pressure at the tympanic membrance. The CM is symmetrical and behaves as if it responds to basilar membrane displacement. The depolarizing phase of the inner hair cell receptor potential exceeds the hyperpolarizing phase with a ratio of about 3:1 in response to sinusoidal stimulation. Below 100 Hz inner hair cell receptor potentials phase lead sound pressure by 180 degrees and their amplitudes increase at a rate of 12 dB/octave. Above 200 Hz their receptor potentials are in phase with CM. The capacitative impedances of the hair cells delay and attenuate the intracellular receptor potentials at frequencies above 178-873 Hz. We conclude that CM is dominated by the responses of outer hair cells, and that a frequencies below 100-200 Hz inner hair cells respond to basilar membrance velocity. Above this they respond to basilar membrane displacement.