The responses of single auditory nerve (AN) fibers in the cat were recorded in response to 25 ms tone pips. Peristimulus time histograms (PSTH) of discharge patterns recorded from fibers with high spontaneous rates (high SRs), show that the discharge rate rapidly adapts to a much lower steady-state level over a 15 ms period with shorter times for units with best frequencies (CFs) greater than 5 kHz. The PSTHs of auditory nerve fibers with low SRs do not show this pattern of rapid adaptation. Differences between the high and low SR populations include higher thresholds, better tuning, and longer latency in the low SR population. The peak-to-steady-state discharge ratio is an increasing function of SR and CF; it varies from 1.0 for fibers with SR = 0 to over 8 for fibers with high SRs and CFs near 10 kHz. This ratio increases with increasing stimulus intensity and stimulus recovery time. The high SR population shows a number of responses to transients which are weak or absent in the low SR population. Increasing the recovery time shortened the latency of both high and low SR AN fibers by as much as 1 ms. A number of other response properties of AN fibers are also reported that are important when interpreting the responses of cochlear nucleus neurons to tone pips.