Glucagon-like peptide-1 (GLP-1) is an insulinotropic hormone secreted from endocrine cells in the gut mucosa in response to meal ingestion. It is an important incretin hormone; mice with a null mutation in the GLP-1 receptor gene develop glucose intolerance. In addition, it inhibits gastrointestinal secretion and motility and is thought to be part of the "ileal brake" mechanism. Perhaps because of the latter actions it inhibits food intake, but intracerebral injection of GLP-1 also inhibits food intake. The insulinotropic effect is preserved in patients with type 2 diabetes mellitus, in whom also glucagon secretion is inhibited. Thus upon i.v. GLP-1 infusion blood glucose may be completely normalised. Because its actions are glucose-dependent hypoglycaemia does not develop. However, GLP-1 is metabolised extremely rapidly in vivo, initially by a mechanism that involves the enzyme dipeptidyl peptidase-IV. It is currently being investigated how GLP-1 or analogues thereof can be employed in practical diabetes therapy. Promising solutions include the development of stable analogues and inhibitors of the degrading enzyme.