The distribution of enteroendocrine cells showing immunoreactivities to four peptides and one amine was examined in the gastric mucosa of the opossum during postnatal development using specific immunocytochemical methods. Gastrin-, BPP-, glucagon-, somatostatin- and 5-HT-immunoreactive cells were identified in the gastric mucosa of the newborn opossum. Gastrin-immunoreactive cells were restricted to the epithelial lining of the pylorus; glucagon-immunoreactive cells were seen only in the epithelium of the fundus. Somatostatin- and 5-HT-immunoreactive cells were found in the epithelium of both fundic and pyloric regions. BPP-immunoreactive cells were the most numerous endocrine cell type seen in the gastric epithelium of the newborn opossum and although found mainly at the confluence of the fundic and pyloric regions, were confined primarily to the fundus in the one week old opossum and all older animals. A marked increase in all five immunoreactive cell types was seen by the end of the first postnatal week. Gastrin-immunoreactive cells were the most numerous at this time and exceeded adult numbers. Both glucagon- and BPP-immunoreactive cells were confined to the fundic glands of older animals and showed the same pattern of decline with age. Somatostatin- and 5-HT-immunoreactive cells showed a shift in population from the fundus to the pylorus with age and together with gastrin-immunoreactive cells were restricted to a narrow zone at the bottoms of the gastric pits and the upper parts of the pyloric glands.