Histamine, a biogenic amine, is involved in allergic reactions and asthma. The involvement of histamine in peptide ulcers is reviewed here. The discovery, distribution, synthesis, catabolism, and pharmacological effects of histamine are briefly described. Histamine actions are mediated by more than one type of receptor. The discovery, development and mode of action of H2-antagonists is discussed. A brief comparison of the clinical profiles (dosage regimen, metabolism and drug interactions) of the four currently used H2-antagonists (cimetidine, ranitidine, nizatidine and famotidine) is given. Furthermore, due to their ability to bind to cytochrome P-450, these compounds have the potential to interfere with the hepatic clearance of other drugs which are also metabolised by the mixed-function oxidase system in man. Therefore, a brief discussion of their adverse effects and drug interactions is included. Modulation of gastric acid secretion, in particular the role of cAMP and the proton pump, is described. Peptic ulcer is a major disease in the Western world and the aetiology and treatment of peptic ulcer are summarised.