We propose to conduct light and electron microscopic studies of the two distinct auditory end-organs in primates, the inner and outer hair cell systems. An attempt will be made to elucidate their synaptic relationships, the differences in innervation of the inner and outer hair cells, and the differences in their projection to auditory centers in the brain. Little is known of the functional significance of the double hair cells system, present in all mammals, and there are surprisingly few studies in primates. The experimental morphological analysis which we further propose depends upon the well known trophic interdependence of nerve cells and the cells they innervate. This phenomenon, which is especially marked in early life, will be exploited in order to determine whether one can selectively destroy inner and outer hairs experimentally by means of appropriate denervation of afferent fibers or of direct or crossed efferent olivocochlear fibers. Clarification of the structure and connections of inner and outer hair cells in primates would be a major extension of contemporary discoveries of cytological and functional aspects of cochlear sensorineural mechanisms, and would strengthen the basic foundations upon which knowledge of mechanisms of deafness must rest.