A number of peptides, closely related in terms of their amino acid sequences, have been found to occur in the gastroenteropancreatic system on the one hand and in the brain on the other. A common range of peptides links the pituitary gland and the placenta and 4 groups of peptides, found in anuran cutaneous glands, are related to the brain and intestinal peptides of mammals. The cells responsible for the production of these peptides, in whatever situation they occur, belong to the APUD ("Amine Precursor Uptake and Decarboxylation") series and they share a number of cytochemical and ultrastructural characteristics which reflect their functional similarities. It was originally suggested that the production of common peptides could be explained on the basis of the origin of the cells from a common ancestor. It is now postulated rather that, being derived at different periods of time from the embryonic epiblast--or perhaps, more accurately, ectoblast--they share a common neuroendocrine programme which is manifested by their possession of the acronymous amine-handling characteristics of the series and whose final expression is determined by microenvironmental circumstances. The APUD concept thus links together a whole spectrum of peptides some of which are hormones, or local hormones, others neurotransmitters and yet others probably exotoxins. Its implications are widespread, and far reaching, affecting most disciplines in the field of medical biology.