Enterochromaffin cells are present in all parts of the gastrointestinal tract from the lower end of the esophagus to the rectum. In the stomach the cells decrease in number from the cardiac end to the pyloric end. In the small intestine an initial cranio-caudal gradient of distribution is later modified so that the density of cells decreases cranio-caudally in the proximal part of the small intestine, but thereafter progressively increases towards the terminal ileum. At the 65 mm stage enterochromaffin cells in the large intestine are least numerous in the cecum and sharply increases in density in a cranio-caudal direction. At the 75 mm and 97 mm stages they are abundant in the cecum; proceeding toward the rectum they show an initial decrease in number followed by a progressive increase. At the 140 mm stage the gradient seen at the 65 mm stage is reversed and the cells now show a uniform cranio-caudal fall in density. At the 220 mm stage, the cells are maximal in the cecum, but the rest of the large intestine shows no definite cranio-caudal gradient. At full-term the pattern is again the same as at the 75 mm and 97 mm stages. In the large intestine of a 13-year-old girl, the pattern of distribution is the same as at the 65 mm stage. Each of the 3 patterns seen at the 65 mm, 75 mm and 140 mm stages may persist into post-natal life. Relative to the rest of the gastrointestinal tract, the total enterochromaffin cell population of the appendix is small and shows no correlation with the high incidence of appendicular carcinoids.