YF476 is clearly a potent and selective gastrin/CCK-B antagonist and should inhibit basal and meal-stimulated gastric acid secretion and enhance gastric emptying of a liquid meal in man. Therefore YF476 might benefit patients with reflux oesophagitis. The compound has been well tolerated in animal toxicity studies at doses in excess of the projected therapeutic dose in patients, and merits studies in healthy volunteers. Extrapolation from data obtained in animals suggests that a single oral dose of about 10mg of YF476 should compare favourably with an oral dose of 150mg of ranitidine in man with respect to effect on basal and food-stimulated gastric acid secretion. Therefore in the proposed study a range of doses of YF476 encompassing that dose will be studied; the exact dose of YF476 will be chosen on the basis of the previous study but the top dose will not exceed 100mg. 24-hour ambulatory gastric pH will be monitored via an intragastric pH electrode (7-10). Although there is variability between subjects with respect to the effects of food and drug treatment on gastric pH, the methodology for measurement of ambulatory gastric pH is robust.