An age-related consecutive morphological analysis of the cochlea has been performed in homozygote (Je/Je) and heterozygote (je/+) jerker mouse mutants. A difference in the time of onset of hair cell pathology was evident between the two, but, when taking place, it showed a similar morphological type of degeneration. The cuticular plate and the stereocilia are particularly vulnerable structures and are the primary sites of damage. The suprastructures on both outer and inner hair cells disintegrate at the same time, irrespective of the level along the basilar membrane. During aging, the je/+ animals showed a progressive age-related impairment of auditory brainstem response thresholds which was correlated semiquantitatively to hair cell pathology.