Rectosigmoid- or colonoscopy biopsies were performed at different levels of the large intestine of 16 adult subjects free from major intestinal disease. Tissue specimens were subsequently examined by electron microscopy. Four different endocrine cell types could be distinguished on the basis of the shape and ultrastructural appearance of secretory granules, of their silver affinity, and of their morphometry. Type I cells (enterochromaffin cells, 60.5%) were argentaffin and presented granules with widely different morphologies, which could correspond to several subtypes. The three other cell types were nonargentaffin. Type II (21.3%) contained granules with a diameter distribution peak of 200 to 300 nm. Type III cells, rare (3.2%), were characterized by small and infrequent granules with a diameter distribution peak of 150 to 200 nm. Type IV cells (7.4%) were characterized by large and round granules with a diameter distribution peak of 300 to 400 nm and resembled the "L cell" of the small intestine. The most endocrine cell-rich portion of the colon was the rectum (179 of 618 cells examined). Type I cells represented approximately 73% of the endocrine cell population from the cecum to the descending colon and decreased to 42% in the rectum. Both the absolute number and the percentage of type II cells increased from the cecum (8%) to the rectum (31%). Type IV cells also increased in the rectum both in absolute number and percentage (14%). The absolute number of type III cells was too small to enable significant differences to be observed.