The distribution and quantification of enteroendocrine cells exhibiting immunoreactivities to nine peptides and one amine were examined in the gastrointestinal mucosa of the adult opossum using specific immunocytochemical methods. In the stomach, 90% of the enteroendocrine cells are confined to the pyloric glands and this region contained 73% of the gastrin-containing cells, 60% of the somatostatin-containing cells and 9% of cells reactive for 5-HT. Enteroendocrine cells showing immunoreactivities to glucagon, pancreatic polypeptide, somatostatin and 5-HT were observed scattered within the oxyntic glands. Only somatostatin and 5-HT positive cells were found in the cardiac glands. Immunoreactivities to CCK, glucagon, gastrin, BPP, somatostatin, secretin, motilin, neurotensin, GIP and 5-HT were observed in the epithelium of the small intestine. Although considerable variation exists in the distribution of individual enteroendocrine cell types along the intestinal tract, nearly equal numbers of enteroendocrine cells were observed in each segment. The percentage of enteroendocrine cells increases distally in the colon. Of the three enteroendocrine cell types present, somatostatin- and 5-HT-immunoreactive cells are evenly distributed, whereas neurotensin-immunoreactive cells increase in numbers distally, resulting in an increase in total number.