Abstract The distribution of glutaminase (GLNase)-like immunoreactivity (IR) in the normal and surgically de-efferented organ of Corti of guinea pig was studied. Primary antisera were against phosphate-dependent GLNase from rat kidney. Indirect immunocytochemical techniques were used; IR was visualized in cryostat sections through immunofluorescence, and through immunofluorescence or with horseradish peroxidase reaction product in surface preparations. Standard microscopy and video-enhanced light microscopy with asymmetric illumination contrast were used. GLNase-like IR was found at inner hair cells (IHCs) in the normal and in the de-efferented organ of Corti, in the tunnel spiral bundle, in tunnel-crossing fibers, in endings high up on outer hair cells (OHCs), in outer spiral bundles, in puncta close to OHCs, and in large, efferent endings at OHC bases. There was no GLNase-like IR at OHCs in the de-efferented organ of Corti. It is concluded that GLNase-like IR is present in auditory nerve dendrites at IHCs and in olivocochlear efferents of the medial system, and that future studies are needed to determine whether also the lateral system of olivocochlear efferents contains GLNase-like IR. A diagram is included depicting the relation between OHCs and efferent nerve endings along the cochlear spiral, showing that in the apicalmost 3 4 turn of the spiral OHCs have no efferent endings.