Serotonin-containing EC cells in human fetal, infantile and adult stomachs both normal and affected by gastritis, were studied by immunohistochemical, electron microscopic and autoradiographic methods. EC cells were sparse in fetal and infantile stomachs, while they occurred in the lower half of the gastric mucosa in adult stomachs showing no atrophic changes and their distribution density was higher than that of D cells. With the progress of chronic gastritis, the number of EC cells gradually decreased, but intestinal type of EC cells appeared in intestinalized gastric mucosa, often showing hyperplasia. Most of EC cells showed argyrophil reaction, but only about 10-20% of them were positive with argentaffin. Epithelial cells with 3H-TdR labeled nuclei were frequently detected in the gastric mucosa where EC cells were sparse or almost absent. Electron microscopically, EC cells had typical electron dense granules in both the normal gastric mucosa and in the intestinal metaplastic glands, but the number of secretory granules was greater in the latter than in the former. These findings suggested that EC cells are preferentially present in the gastric mucosa with a small number of labeled nuclei and have morphological heterogeneity.