Deterioration followed by recovery of behavioural absolute threshold and frequency selectivity has been observed in guinea pigs following kanamycin administration of 200 mg/kg body weight daily for 16 days. Deterioration in function consistently follows a high-to-low frequency pattern and recovery generally occurs at the lowest of the high (8-32 kHz) frequencies affected. The degree of recovery is related to the magnitude of the threshold elevation; where large (40-45 dB) elevations occur initially, the process appears to be partial since threshold recovers only to within 5-12 dB of pre-administration levels. In instances where smaller threshold elevations (5-20 dB) take place initially, recovery can sometimes be complete. However, when threshold elevations of over 50 dB occur, no recovery is apparent. Recovery is relatively slow, taking place over periods of up to 100 days post-kanamycin administration. Hair cell counts have established that the threshold elevation which remains in instances of partial recovery is not related to a reduction in hair cell numbers at the light microscope level.