Exposure of the guinea pig cochlea to loud pure tones caused a dramatic swelling of afferent dendrites beneath the inner hair cell (IHC). This swelling occurred in a restricted region of the cochlea basalward of the exposure frequency location. For a 110 dB tone swelling was just detectable in 1 micron sections for a 18 3/4 min exposure and was clearly visible after a 22 1/2 min exposure. Swelling was reversible. Exposures which caused swelling produced a loss in sensitivity of the flat low frequency 'tail' of the frequency-threshold curves of single auditory neurons whose most sensitive frequency was a 1/2 octave higher than the exposure frequency. The findings are consistent with the notion that dendritic swelling causes a non-selective decrease in sensitivity to all frequencies of sound.