Whole nerve compound action potentials (APs) are used to determine AP threshold curves of individual chinchillas. Initially, each animal is implanted with a chronic round window electrode that is connected to a small plug fastened to the skull. After recovery from surgery the animal is suspended in an anechoic chamber and presented with free‐field tone‐bursts. Computer‐averaged APs (N= 128), monitored on an oscilloscope, are used for visual detection ofN1, thresholds for several tone‐burst frequencies from 0.5 to 20 kHz. In this manner, a threshold curve is determined on the basis of data accumulated over at least four separate days. Animals considered to have normal thresholds are then placed on a kanamycin regimen. Kanamycin, an ototoxic antibiotic, produces a lesion that progresses through the cochlea from base to apex, preferentially destroying outer hair cells; hence, the first effects of the drug are manifest as a threshold shift at higher frequencies. Animals receiving kanamycin injections are monitored daily in the same manner that established their initial threshold curves. The extent of cochlear damage is reflected in postkanamycin threshold curves. [Work supported by NINCDS.]