The present study was performed to examine the distribution and distinct morphology of the serotonin-containing enterochromaffin (EC) cells in the rat distal colon by immunohistochemical and electron microscopic methods. Serotonin-immunohistochemistry revealed that most of the serotonin-immunoreactive EC cells possessed extended cytoplasmic processes. In particular, the immunoreactive EC cells with long processes located along the body of the crypt were characterized by their bipolar processes comprising one with the terminal swellings extending vertically down to the basal crypt and the other running up along the luminal side - in many cases, with the apical ends reaching the glandular lumen. Moreover, a few EC cells had long processes which resembled neuronal processes with varicosities. Electron microscopic observations revealed rod-like, tortuous, oval, or round small pleomorphic granules in the long processbearing EC cells. The cell bodies and processes directly faced the crypt epithelial cells - including the enterocytes and goblet cells on one side and the basement membrane on the opposite side. The accumulation of the granules sometimes appeared within the cytoplasm on the side of the epithelial cells. These findings suggest that serotonin is released from the long processes of the EC cells and directly acts in a paracrine fashion on the crypt epithelial cells to secrete electrolytes and fluids into the colonic lumen. The long cytoplasmic processes of the EC cells may be a major contributor to the serotonininduced secretory events in the rat distal colon.