Serum gastrin was measured by radio-immunoassay during postvagotomy insulin tests in 33 patients. In 11, the study was preceded by propranolol 20 mg, orally. Gastric acid output was lower in the group pretreated with propranolol. There was no difference in serum gastrin between Hollander ‘positive’ versus Hollander ‘negative’ insulin tests. Propranolol affects insulin-stimulated gastric secretion in ways other than by affecting total serum gastrin.