The fine structure of the afferent and efferent recepto-neural junctions at the outer hair cells of the cat cochlea (outer receptors) is described. Type I recepto-neural junctions (afferent synapses) consist of small type I nerve endings and the presynaptic receptor cytoplasm. The latter presents scanty synaptic vesicles and numerous smooth canaliculi in direct continuity with adjacent subsurface cisternae and with acanthous vesicles. Type II recepto-neural junctions (efferent synapses) are composed of large type II endings and the postsynaptic receptor cytoplasm. The type II terminals are filled with synaptic vesicles, granular vesicles and mitochondria. The receptor component possesses subsurface cisternae beneath the synaptic plasmalemma. Free microglobules and filaments are seen across the synaptic cleft. The probable significance of the structures described is discussed.