Immunohistochemistry as well as immunohistofluorescence were used to investigate the distribution of the neurotrophin-3 (NT3) in the adult rat cochlear nucleus. We found a widespread distribution of NT3 immunolabeled neurons throughout the three divisions of this nucleus. NT3-like immunoreactivity was clearly population-specific, with some cell groups heavily (various small neurons and granule cells) or moderately (large neurons of the ventral cochlear nucleus) stained, while others remained negative (a major fraction of medium and large neurons of the dorsal cochlear nucleus). Double-labeling experiments were performed using antibody against the glial fibrillary acid protein, a classic marker for mature astrocytes. This colocalization study revealed that NT3 immunoreactivity was also present in a subpopulation of astrocytes, particularly in the glia limitans and their projections. Numerous small cells also colocalized NT3 together with the glial marker in the granule cell domain and in the molecular cell layer of the dorsal cochlear nucleus. These results suggest that NT3 may exist in widespread populations of adult cochlear nucleus neurons as well as in glial cells. This abundant distribution of NT3-like immunoreactivity implies that this neurotrophin may have an important role in the continued maintenance of mature cochlear nucleus and makes it an attractive candidate for playing a role in regulation or stabilization of neuronal circuits in this nucleus.