In the course of an ultrastructural study on peroral gastric biopsy specimens that were obtained from patients with chronic atrophic gastritis, peculiar pathological changes of endocrine cells were observed and correlated with functional and hormonal data on the patients. An increased number of G (gastrin) cells was found in hypergastrinemic patients. These cells could be divided into a "light" (probably hyperfunctioning) and a "dark" (probably exhausted) type. The light type of cell was prominent regardless of the concomitant gastrin blood levels. The G cells found within the fundic region were always localized within the areas of pyloric metaplasia. Focal micronodular proliferation of antral enterochromaffin cells (EC) was often seen. A proliferation of the closed type of endocrine cells of the fundic mucosa was observed only in patients with elevated gastrin concentrations. In the present study, these cells were identified as enterochromaffin-like cells (ECL). No substantial changes were found in the D and D1 cells. The endocrine cells seen in metaplastic intestinal epithelium exhibited ultrastruct characteristics of at least three different types of intestinal endocrine cells (EC, L, and S cells).