The onset and development of neuron-specific enolase (NSE) and calbindin immunoreactivities were studied in the inner ear of human fetuses aged from 6-7 to 14 weeks of gestation. NSE occurred very early in ganglion neurons. Its appearance in vestibular sensory cells at 8 weeks coincided with the formation of the first afferent synapses, and showed an apex/base gradient in the cristae. Calbindin was found in vestibular ganglion neurons at 6-7 weeks and in the cochlear ganglion neurons at 8-9 weeks. Vestibular sensory cells and the whole ventral wall of the cochlear duct were stained from 8-9 weeks. At 14 weeks, calbindin staining occurred only in the sensory cells of the cochlear neuroepithelium. Non-neuronal secretory structures, i.e. Kölliker's organ and some cells of the transitional zone of the utricle, were also reactive. Staining appeared in Kölliker's organ with a base to apex gradient and disappeared from it with an internal to external gradient. Calbindin appeared in vestibular sensory cells later than NSE staining, synapse formation and sensory hair bundle differentiation. By contrast in the cochlea, calbindin staining appeared in the neuroepithelium before sensory cell differentiation, but remained only in the hair cells after they had differentiated and been contacted by the afferent fibers.