Behaviorally determined auditory thresholds of chinchillas were monitored before, during and after daily administration of kanamycin sulfate (200 mg/kg per day). Drug treatment was terminated as soon as a shift in high-frequency thresholds was observed. In surviving animals, hearing loss spread toward lower frequencies and stabilized after eight to nine days. The stable hearing loss was limited, on the average, to frequencies above 3 kHz. The magnitude of the loss was constant across frequencies and averaged about 40 dB. Virtually complete outer hair cell loss was seen in the basal portion of the cochleae of all surviving animals.