The relationship between cochlear damage and loss of hearing sensitivity was studied in chinchillas exposed to noise levels common to industrial environments. The animals were exposed to tape recorded industrial impact noise at 125 decibels peak pressure presented once every 2 seconds for 8 hours per day. Work week exposures consisted of 5 consecutive exposure days followed by 2 nonexposure days, and groups of animals were exposed for 1, 2, and 4 work weeks. Hearing sensitivity was assessed by auditory evoked response (AER) thresholds measured from the area of the inferior colliculus at 250, 500, 1000, 3000, 4000, and 8000 hertz (Hz) pure tone signals administered as 20 millisecond bursts. The animals were sacrificed after AER threshold determination, and cochlear morphology was examined by scanning electron microscopy. Damage was determined by ciliary disarray, ciliary fusion, missing cilia, ruptured sensory cell membrane, and missing hair cells. One week exposed animals showed severe to profound outer hair cell damage and an average 21 decibel (dB) reduction in the AER threshold at 1000, 2000, and 8000Hz. Increasing amounts of inner hair cell damage were noted in the 2 and 4 week exposure groups. Animals exposed for 2 weeks had an average 19dB loss in the AER threshold at 1000 and 2000Hz, and maximum losses of 24dB for 1000 and 2000Hz tones and 18dB for 500 and 4000Hz tones were observed in the 4 week exposure group. The 4 week exposure group showed a hair cell damage pattern involving the first and third rows of cells without significant damage to the second row. Morphological alterations in the basal half of the cochlea were correlated with changes in AER thresholds above 1000Hz for each of the exposure groups. The authors conclude that the most apical 65 percent of the cochlea is the most sensitive to the peak energy output of the impact noise between 500 and 2000Hz.