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Since the source and disposition of serotonin is still unclear in non-neuronal cells, we investigated the cellular expression of serotonin transporter (SERT) in mouse adrenal chromaffin cells, mast cells and enterochromaffin cells, all of which contain serotonin in their secretory granules. Immunohistochemistry using SERT antibody in the adrenal medulla demonstrated an intense immunoreactivity in the plasma membrane of adrenalin (A) cells, but not in that of noradrenalin (NA) cells. Electron-microscopically, the expression of SERT appeared to occur mainly on the lateral plasma membranes of A cells facing adjacent A cells. Peritoneal mast cells also displayed a selective immunoreactivity for SERT along the plasma membrane, although the intensity varied cell to cell. In the tongue, stomach, nasal mucosa and trachea, connective tissue-type mast cells were immunoreactive, but not mucosal-type mast cells. Gastrointestinal epithelial cells including enterochromaffin cells were free of the immunoreactivity. These findings suggest that SERT is involved in the uptake of serotonin in adrenal A cells and connective tissue-type mast cells.