Cochlear microphonic (CM) potentials were recorded throughout the development of auditory function in the Mongolian gerbil. CM responses were first recorded at 12 days after birth (DAB), with thresholds exceeding 103 dB SPL. CM thresholds subsequently improved rapidly in a parallel fashion across the responsive frequency range, achieving adult levels by 18 DAB. There was no evidence from CM thresholds of a preferential rate of maturation for either the high or low frequency ranges. CM responses to suptrathreshold stimulation were also studied throughout development. At all responsive ages, CM input-output functions increased logarithmically to a saturating maximum value and then declined. Maximum CM responses increased continuously throughout development in a parallel fashion across all frequencies. The dynamic range of the CM input-output functions (the intensity interval from CM threshold to maximum CM value) also developed in a parallel manner across frequencies, but reached mature assymptotic values by 16 DAB. The results suggest that throughout the adult CM frequency range cochlear hair cell function develops simultaneously.