Many neurotransmitter candidates have been identified in the cochlea and cochlear nucleus with the use of immunocytochemical techniques. Choline acetyltransferase immunoreactivity suggests acetylcholine as a transmitter of medial and lateral efferent systems in the cochlea. Immunoreactivities to enkephalins, dynorphins, calcitonin gene-related peptide, and tyrosine hydroxylase (a marker for dopamine) are also found in lateral efferents. Choline acetyltransferase, enkephalin, and dynorphin immunoreactivities are co-contained in neurons of the lateral system. In the anteroventral cochlear nucleus, the inhibitory amino acid transmitters, gamma aminobutyric acid (GABA), and glycine, as well as the presumed excitatory amino acid transmitter of the auditory nerve, have been directly or indirectly localized, immunocytochemically, to discrete populations of terminals on spherical cells with distinct morphologic characteristics.