The gastric secretory response to impromidine (SKF 92676), a potent selective histamine H2 agonist, has been studied in 40 patients with peptic ulcer disease or ulcer-like dyspepsia. Peak acid output following the intravenous infusion of impromidine, 10 μg kg-1·h-1, or pentagastrin, 6 μg kg-1h-1, was not significantly different. This was also the case when the two drugs were administered subcutaneously at the same dose. Impromidine was well tolerated with only minor effects on the cardiovascular system both in 10 younger patients and in 20 subjects over the age of 55 years with mild to moderate cardiovascular disease. Impromidine can be considered an important tool for further investigation of the role of histamine at the H2-receptor site in man.