The role of 'conditioning' exposures as moderators of hearing loss produced by exposure to a higher level noise was explored using chinchillas. Monaural chinchillas were exposed to an octave band of noise centered at 0.5 kHz at 95 dB for six hours/day for ten days. The subjects were allowed to recover to pre-exposure sensitivity and at five days after the last exposure they were re-exposed to the same noise at 106 dB. Thresholds recorded at various time intervals following the second exposure were compared with those recorded in a control group exposed only to the higher level noise. The experimental animals were found to have less threshold shift at all stages of recovery. Results are discussed in the light of results of other related studies and possible mechanisms involved are hypothesized.