The regional and topographic distribution of endocrine cells in the human intestine was examined by immunohistochemistry. The frequency of endocrine cells was greatest in the small intestine with the rectum next in order. The duodenum and jejunum harbored a large number of different endocrine cell types; the spectrum of cell types gradually narrowed distally in the intestine. 5-Hydroxytryptamine-containing enterochromaffin cells were present in all regions of the intestine and comprised the single largest endocrine cell population. In addition, a minor proportion of these cells contained substance P. The second largest cell population consisted of the glicentin cells, which were notably numerous in the ileum and colon. The somatostatin cells also occurred throughout the digestive tract. Cells storing cholecystokinin, motilin, secretin, or gastric inhibitory polypeptide were more numerous in the proximal and middle small intestine than distally. Gastrin cells were few and occurred in the proximal duodenum only. Other cells in the small intestine reacted with antiserum directed against the common C-terminus of gastrin and cholecystokinin. The number of these cells greatly exceeded the sum of cells reactive to gastrin-specific or cholecystokinin-specific antisera. Cells displaying beta-endorphin, pro-gamma-melanocyte-stimulating hormone, or beta-lipotropin immunoreactivity, or a combination of these, were found in the small intestine. Cells storing neurotensin, glicentin, substance P, or pro-gamma-melanocyte-stimulating hormone increased in number distally in the small intestine. Enterochromaffin cells, glicentin cells, and somatostatin cells were the predominant endocrine cell types in the colon and rectum. The majority of the glicentin-immunoreactive cells also contained glucagon and pancreatic polypeptide-like immunoreactivity. Endocrine cells in the large intestine often possessed basal processes.