The endocrine cells of the digestive system (entero-endocrine cells of gastro-intestinal epithelia and Langerhans' islets of the pancreas) and the chemical messengers produced by them constitute a complicated and complex system. The physiological function of this system is the regulation of all processes related to digestion and resorption, and to homeostasis of carbohydrate metabolism. Using morphological and histochemical features of this cellular community, the present review deals with amines and amine metabolism, polypeptides and their immunohistochemical identification, and with the modes of action of enteric and pancreatic hormones. Special attention is paid to the significance of amine precursor uptake and decarboxylation (APUD), to immunohistochemical methodology and the interpretation of immunohistochemical findings, and to local regulatory mechanisms, especially paracrinia. Finally, unifying concepts for the integration of these cells and similar endocrine cells of other organs into a common system are considered.