In most mammals, the cochlear efferent innervation originates from separate groups of neurons located in the lateral and medial parts of the superior olivary complex which project, respectively, to the regions beneath the inner and outer hair cells of the organ of Corti. Because of methodological limitations, previous studies have been unable to distinguish between the development of lateral vs medial efferent fibers. In order to fill this gap, the goals of this project are to determine, with light and electron microscopic fiber tracing techniques, the chronology of postnatal developmental changes in morphology and neurotransmitter immunocytochemistry that occur in selectivity labeled lateral and medial olivocochlear axons in the pigmented Long Evans rat. The specific aims of this project are: 1) to clarify some of the remaining issues concerning the terminal arbors and postsynaptic targets of lateral olivocochlear (LOC) and medial olivocochlear (MOC) axons in the organ of Corti of adult rat so as to provide the necessary standard against which developmental changes will be compared, 2) to describe the postnatal development of terminal arbors and the postsynaptic targets of LOC and MOC axons, as well as of afferent fibers, in an age graded series of rat pups, 3) to determine if LOC and MOC axons which are immunopositive for antisera to choline acetyltransferase (ChAT) or gamma amino-butyric acid (GABA) have distinct developmental or connectional features, and 4) to describe the postnatal development of the terminal arbors and postsynaptic targets of MOC efferent fibers in the kitten organ of Corti. These studies should lead to a better understanding of the developmental progression and regression of the two systems of cochlear efferent fibers and the principal afferent neurons of the cochlea.