Immunocytochemical techniques were used to investigate the distribution and co-localization of neuron-specific enolase (NSE) and serotonin (5-HT) in the skin of the conger eel, Conger conger. NSE and 5-HT immunoreactivity were found in Merkel cells; these cells were also identified at the electron-microscope level by the presence of characteristic granules and their association with an intraepithelial nerve ending. For the first time, it was demonstrated that Merkel-cell granules of vertebrate skin exhibit an immunoreaction with 5-HT. The production of amines may indicate that the Merkel cells of C. conger have both secretory capabilities and transduction functions. However, immunocytochemical investigation of the synaptic zones at the electron microscope level will be necessary to confirm this hypothesis. The present histochemical results suggest that NSE and 5-HT may be marker substances for Merkel cells, and that immunocytochemistry is a useful tool for the light-microscopic localization of these cells.