The immunofluorescence technique has been applied to the organ of Corti from developing guinea pigs to detect the immunoreactions to antibodies directed against Met-enkephalin and Met-enkephalin-Arg6-Gly7-Leu8. Four stages of gestation were studied (30, 41, 50 and 62 days). On days 30 and 41 of gestation, no specific fluorescence to both antibodies was seen in the organ of Corti. On day 50, the inner spiral bundle and the tunnel spiral bundle, two areas displaying anti-enkephalin immunoreactivity in the adult, showed a specific immunofluorescence to either the anti-Met-enkephalin and the anti-Met-enkephalin-Arg6-Gly7-Leu8 antibodies. On day 62, these two structures showed an immunofluorescence whose intensity was slightly increased compared with that seen on day 50. In no case, was an immunoreaction in the outer hair cell area seen. These results suggest that the enkephalins, which have been proposed as putative neurotransmitters or neuromodulators of neurons belonging to the lateral olivocochlear system, are present in the organ of Corti at about the time of onset of cochlear function. Thus, the enkephalin-containing lateral efferents may play a role in the early control of cochlear potentials.