Growth and decay of temporary threshold shifts (TTS), small permanent threshold shifts (PTS), and cochlear injury are described for two chincillas exposed to an octave band of thermal noise (0.3–0.6 kHz) at a sound-pressure level (SPL re 0.0002 μbar) of 100 dB for seven days. TTS grew with the duration of the exposure for 24–48 h and remained constant thereafter. After termination of the exposure, TTS decayed slowly and reached near-zero values in about five days. Three months after exposure there were small PTSs (less than 10 dB) for some test tones, reduced cochlear potentials, and clear losses of external hair cells in the second and third turns of the cochlea. Observations of TTS for identical exposures limited to a duration of 193 min are also described.