The development of the auditory brainstem response was studied to quantitatively assess its dependence on stimulus frequency and level. Responses were not observed to stimuli > or =16 kHz on P12, however, the full range of responsive frequencies included in the study was observed by P14. Response thresholds were high on P12, exceeding 100 dB SPL for all stimuli tested. The rate of threshold development increased progressively for stimulus frequencies between -2 and 10 kHz, with the most rapid changes occurring at frequencies >10 kHz. Adultlike thresholds were observed by P18. Response latencies and interpeak intervals matured rapidly over the course of the second and third postnatal weeks and did not achieve adultlike characteristics until after P18. Latencies of higher-order peaks were progressively and sequentially delayed relative to wave I. Wave I amplitudes developed nonmonotonically, growing during the first 24 days and stabilizing at adult values by approximately P36. Slopes of wave I amplitude-and latency-level curves were significantly steeper than those of adults during the neonatal period and the outcome of input-output analyses, as well as frequency-specific maturational profiles, support developmental models in which function initially matures in the mid-frequency range and proceeds, simultaneously, in both apical and basal directions.