A morphometric analysis of the efferent innervation of the outer hair cells in the organ of Corti of one chimpanzee was performed. There was a wide variability in the size of efferent endings which ranged from approximately 0.1 to 4.5 microns 3 in volume. Based on the size distribution of endings, a volume of 1.0 microns 3 was chosen to divide efferent endings into two groups, 'large' and 'small'. The incidence of large efferent fibers decreased from base to apex and from the first to third row of each turn, whereas the incidence of small efferents increased from base to apex and from the first to third row in each turn. This data was interpreted to suggest that at least two types of efferent endings may exist at the base of outer hair cells of the organ of Corti in the chimpanzee.