A number of peptide hormones and hormone candidates were studied by immunocytochemistry with respect to their appearance and distribution in the developing porcine gastroenteropancreatic region. The hormones of the pancreatic islets were the first to appear. At 4 weeks' gestation (the earliest stage studied), glucagon, insulin, and somatostatin cells occurred in the dorsal pancreatic primordium, whereas pancreatic polypeptide cells occurred in the ventral primordium. At this stage, the pancreatic primordia were made up of strands of endocrine cells, and no ducts or acini were seen. Subsequently, the endocrine cells were separated by the growing exocrine parenchyma; at still later stages, they aggregated in small nests. Not until birth did they form mantle islets with insulin cells in the central core and the other endocrine cell types on the outside. Gastrin cells appeared in the stomach at the 4-wk stage; somatostatin cells appeared about 1 wk later. In the intestines, these two cell types appeared at the 6-wk stage. Cells displaying glucagon immunoreactivity were the first endocrine cells to appear in the intestine. They occurred in the upper small intestine at the 4-wk stage; they later disappeared from this location but appeared instead in the lower small intestine and colon where they remained. Secretin, cholecystokinin, motilin, gastric inhibitory peptide, and neurotensin cells all appeared at the 6-8-wk stage, and were restricted to the small intestine throughout development. Enkephalin immunoreactive cells appeared late during ontogeny (at the 13-15-wk stage) in both the gut and pancreas. Still later (15-17-wk stage), dense accumulations of endocrine cells (Segi's cap) were occasionally observed on the top of villi in the upper small intestine; these accumulations consisted mainly of somatostatin, cholecystokinin, and gastric inhibitory peptide cells. In view of the early appearance of many gastroenteropancreatic endocrine cell types in fetal life, a functional significance of gastroenteropancreatic hormones in the early development of gut and pancreas is likely.