The aim of the present study was to examine the role of the olivocochlear system in auditory processing by examining the long-term effects of cochlear de-efferentation on auditory nerve response properties in adult chinchillas. Spontaneous rates, response thresholds, tuning curves, discharge rate-level functions, and adaptation of single auditory nerve fibers were measured in chinchillas with complete cochlear de-efferentation produced by sectioning the olivocochlear bundle in the internal auditory meatus. De-efferentation was verified as successful on the basis of acetylcholinesterase staining of surface preparations of the organ of Corti. Following chronic de-efferentation, there was a striking decrease in spontaneous rate, consistent with earlier observations in cats. In addition, the present study shows that complete de-efferentation results in: (1) increased driven discharge rates and decreased dynamic range of discharge rate-level functions, (2) larger onset-to-steady state ratio of discharge rate at moderate intensities, and (3) a hypersensitive tail of the tuning curve. These effects, largely confined to neurons that were most sensitive to frequencies between 2-8 kHz, indicate that the cochlear efferent system is important in maintaining normal function (e.g., frequency and intensity selectivity) of the auditory periphery by modulating auditory nerve fiber response properties.