Zinc-containing neurons are cells which sequester zinc in the vesicles of their axonal boutons; such zinc-containing fiber systems have been previously shown to innervate many limbic and cerebrocortical brain regions. The present study of rats and mice shows that zinc-containing axons also innervate the cochlear nuclei, forming two morphologically-distinct projection systems. One zinc-containing pathway innervates the molecular stratum of the dorsal nucleus, supplying a diffuse, even band of neuropil staining throughout the stratum. The other pathway projects sparsely to the various small cell (granule cell) regions of the nuclei where the zinc-positive elements form scattered clusters and threads of bouton-like puncta amidst the granule neuron somata. Preliminary observations indicate that the pattern is the same in the cat as in the rat and mouse.