The normal pancreatic D cell has been well recognised since 1931, but its secretory product has not been identified with certainty. Combined immunocytochemical methods, and histological methods for endocrine granules, have demonstrated that it contains a product with G.H.-R.I.H.-like immunoreactivity. G.H.-R.I.H. has also been localised in the upper gastrointestinal tract, tentatively in the morphologically similar D cell of the Wiesbaden classification. The potent actions of this peptide on pancreatic and gastrointestinal function suggest that it may play an important role in digestive physiology.