The mammalian inner ear loses its sensory cells with advancing age, accompanied by a functional decrease in balance and hearing. This study investigates oxidant stress in the cochlea of aging male CBA/J mice. Glutathione-conjugated proteins, markers of H₂O₂-mediated oxidation, began to increase at 12 months of age; 4-hydroxynonenal and 3-nitrotyrosine, products of hydroxyl radical and peroxynitrite action, respectively, were elevated by 18 months. Immunoreactivity to these markers was stronger in the supporting cells (Deiters and pillar cells) than the sensory cells and appeared later (23 months) in spiral ganglion cells and in the stria vascularis and spiral ligament. Conversely, antioxidant proteins (AIF) and enzymes (SOD2) decreased by 18 months in the organ of Corti (including the sensory cells) and spiral ganglion cells but not in the stria vascularis. These results suggest the presence of different reactive oxygen species and differential time courses of oxidative changes in individual tissues of the aging cochlea. An imbalance of redox status may be a component of age-related hearing loss.