Individual afferent and efferent nerve fibers were identified and traced in Golgi-impregnated cochleas of cats from newborn to one month old. Afferent radial fibers project radially without varicosities to terminate at the base of one or two inner hair cells. Outer spiral fibers have both radial and spiral orientations within the organ of Corti, do not form varicosities while crossing the base of the tunnel, and spiral for long distances in the outer spiral bundles. They contact many outer hair cells of more than one row both en passant and by small terminal branchlets. Two separate groups of efferent fibers are identifiable. Thin efferent fibers with many large varicosities spiral for long distances in the inner and tunnel spiral bundles; varicosities in the inner spiral bundle may contact radial afferent fibers or hair cells, depending on age. Thick radial efferent fibers course radially through the tunnel spiral bundle and across the upper part of the tunnel, often in fascicles. They contact a few outer hair cell bases by large terminals. The spiral expanse of the terminals is limited. These fibers are most common in the more basal turns of the organ. The present results confirm the anatomical separation of radial and spiral afferent fiber systems and identify two separate efferent populations beyond the neonatal period in the cat. The major features of afferent innervation discernible in Golgi-impregnated cochleas are present at birth, although some simplification of afferent fibers probably occurs during the first postnatal week. In contrast, the efferent fiber pattern undergoes important changes during the first few weeks after birth. In mature animals, the fine spiral efferents probably contact only afferent fibers, whereas the thick radial efferents may contact both outer hair cells and spiral afferent fibers. The possibility that some individual efferents branch to both inner and outer hair cell regions in the older cats cannot be ruled out.