During development in the cochlea, NCAM is present on all neuronal systems--radial fibres, inner spiral bundle, inner pillar bundle (described by Sobkowicz & Emmerling, 1989), and outer spiral fibres. In cochleae from E17 mice, NCAM-positive puncta, possibly associated with growth cones, dot the area of the inner spiral bundle. The region overlying the basilar membrane is also NCAM positive. Until about P7, NCAM immunoreactivity surrounds the inner and outer hair cells. During the second postnatal week, NCAM no longer covers the hair cell surfaces. At this time, NCAM is present on the inner spiral and inner pillar bundles. In the outer hair cell region, immunoreactivity concentrates in neuronal endings with 'Y'-like or 'dot'-like appearances typical of cholinergic efferents. At a time when neuronal fibres are regenerating in the cochlea of the Bronx waltzer mutant mouse, many NCAM-positive puncta dot the regions of the inner spiral bundle and inner spiral sulcus, suggesting that these puncta correspond to growth cones. In organotypic culture, hair cells and nerve fibres retain their immunoreactivity up to 7 days. In conclusion, NCAM is present on the surfaces of neurons and on their cochlear targets at the time when nerve-target recognition is proceeding. NCAM disappears from targets during the time of maturation of efferent synapses. NCAM is also present along the growth path of the nerve fibres. These studies suggest that NCAM is properly positioned in the cochlea for possible roles in axon guidance and nerve-target interactions.