Somatostatin is present in the gastrointestinal tract in appreciable amounts. The highest concentrations of the polypeptide are found in the stomach, the upper small intestine, and the pancreas. Within the gastrointestinal tract, somatostatin inhibits various functions, including endocrine and exocrine secretion, motility, blood flow, absorption, and growth. The polypeptide regulates these functions by endocrine, paracrine, neurocrine or luminal mechanisms. Abnormalities of endogenous somatostatin have been implicated in several gastrointestinal disorders, including the somatostatinoma syndrome, antroduodenal D-cell hyperplasia, peptic ulcer, obesity, and liver cirrhosis. Because of its potent inhibitory effects, somatostatin or somatostatin-analogues have been used as therapeutic agents in various clinical conditions, such as upper gastrointestinal haemorrhage, endocrine pancreatic tumours, gastrointestinal and pancreatic fistulas, pancreatitis, secretory diarrhoea, and dumping syndrome. The recent availability of the synthetic long-acting somatostatin-analogue SMS 201-995 (Sandostatin) has greatly facilitated the therapeutical application of somatostatin-polypeptides.