The effect of impact noise on hearing threshold was investigated in a parametric study which manipulated frequency, level, and interstimulus interval (ISI). The chinchilla pinna passively amplifies acoustic stimuli between 2 and 6 kHz suggesting greater effects in that frequency range. Exponentially decaying pulses (t = 40 ms) with carrier frequencies of 1, 3, and 8 kHz were presented at peak pressure levels of 114, 117, 120, and 129 dB SPL and ISI of 1, 10, and 100 s. The chinchillas were exposed for 8 h, with one animal per condition. Hearing function was assessed by measuring auditory-evoked potentials from tonebursts at 0.5, 1, 2, 4, and 8 kHz from implanted electrodes. The data indicate a small (<10 dB) permanent threshold shift (PTS) in the 10- and 100-s ISI conditions. For the 1-s ISI conditions, the animals exposed at 129 dB experienced more than 25 dB PTS at high frequencies. Animals exposed at 120 dB exhibited large temporary threshold shifts (TTS) while the animals exposed at lower levels had smaller TTS and minimal PTS. Stimulus level and ISI had significant effects on the PTS while frequency content was not significant.