Trained chinchillas were exposed to 6 h of noise followed by 18 h of quiet for nine days. Thresholds (0.5-8.0 kHz) were measured immediately before and after each day’s exposure. The decay of threshold shift after the ninth day was followed until stable thresholds were again observed. This procedure was repeated for six levels (57–92 dB SPL) of an octave-band noise centered at 4.0 kHz. The threshold shift measured after 4 min of quiet (TS4) appears to reach an asymptotic level (ATS4) after the first or second exposure. ATS4, measured at frequencies exhibiting greatest shift (5.7 kHz), increases with the level of the noise with the same slope (1.7 dB/dB) for the daily 6-h exposures as for nearly continuous exposures. ATS4 is smaller for 6 h than nearly continuous exposures by about 5 dB and this difference can be explained by an equivalent-power hypothesis. The decay of threshold shift was nearly complete after 18 h of quiet for the lowest levels of noise, while it was nearly complete only after 3–5 days for the intermediate levels of noise. The decay of threshold shift was never complete and small amounts of permanent threshold shift were observed for the highest levels of noise.