The presence of serotonin in the Merkel cells of pig snout epidermis was investigated by the peroxidase-antiperoxidase immunohistochemical technique. Serotonin-like immunoreactive Merkel cells were found in groups located at the base of epidermal rete pegs and in the external root sheath of sinus hair follicles (vibrissae). Immunoreactivity was stronger on the basal side of the Merkel cells, where dense-cored granules are most numerous. Neither the nerve terminal associated with the Merkel cell nor the neighbouring epidermal cells were immunostained. These results are the first evidence of serotonin-like immunoreactivity in mammalian Merkel cells. The fact that immunoreactivity is strongest in those parts of the Merkel cells with the highest granule density suggests that in these cells serotonin is probably localized in the dense-cored granules.