Kv3.1b channel protein is widely distributed in the mammalian auditory brainstem, but studies have focused mainly on regions critical for temporal processing, including the medial nucleus of the trapezoid body (MNTB) and anteroventral cochlear nucleus (AVCN). Because temporal processing declines with age, this study was undertaken to determine if the expression of Kv3.1b likewise declines, and if changes are specific to these nuclei. Immunocytochemistry using an anti-Kv3.1b antibody was performed, and the relative optical density of cells and neuropil was determined from CBA/CaJ mice of four age groups. Declines in expression in AVCN, MNTB, and lateral superior olive (35, 26, and 23%) were found, but changes were limited to neuropil. Interestingly, cellular optical density declines were found in superior paraolivary nucleus, ventral nucleus of the trapezoid body, and lateral nucleus of the trapezoid body (24, 29, and 26%), which comprise the medial olivocochlear (MOC) feedback system. All declines occurred by middle age (15 months old). No age-related changes were found in the remaining regions of cochlear nucleus or in the inferior colliculus. Contralateral suppression of distortion-product otoacoustic emission amplitudes of age-matched littermates also declined by middle age, suggesting a correlation between Kv3.1 expression and MOC function. In search of more direct evidence for such a correlation, Kv3.1b knockout mice were examined. Knockouts show poor MOC function as compared to +/+ and +/- genotypes. Thus, Kv3.1b expression declines in MOC neurons by middle age, and these changes appear to correlate with functional declines in efferent activity in both middle-aged CBA mice and Kv3.1b knockout mice.