Gastrin releasing peptide (GRP) has proved to be a particularly valuable tool in detecting disturbances of gastric secretory function associated with duodenal ulcer disease and Helicobacter pylori infection, and it has furthered understanding of the pathophysiology of these conditions. Its attractiveness lies in the fact that it simultaneously activates many physiological control processes, both stimulatory and inhibitory. This facilitates the detection of a defect in any of the many controls involved in regulating biological function. Other gastrointestinal functions such as gall-bladder contraction, pancreatic secretion and gastrooesophageal motility are also subject to complex regulatory controls, and GRP may also be of value in investigating disturbances of these processes.