With light and electron microscopy, γ-aminobutyric-acid (GABA)-like immunoreactivity was examined in the guinea pig organ of Corti. In whole-mount preparations, although GABA-like immunoreactivity was present within efferent components in all turns of the cochlea, it was more intense apically. At the ultrastructural level, GABA-like immunoreactivity was clearly restricted to the efferent system, appearing in axons of the inner and tunnel spiral bundles, in axons crossing the tunnel of Corti, and in terminals filled with numerous labeled vesicles synapsing on outer hair cells