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 remarkably well tolerated in animal toxicity studies at doses well in excess of the projected therapeutic dose in patients, and merits first administration to healthy volunteers. That study using the oral route of administration is described here. Extrapolation from data obtained with pentagastrin in animals suggest that single doses of less than 1mg of YF476 should be active in man. However, extrapolation from data obtained in comparative studies with the H2-antagonists and with omeprazole in animals suggest that the therapeutic dose in patients with reflux oesophagitis will be larger, in the region of 10mg. A range of single doses will be used in this study. The maximum dose will be 10 times more than the expected therapeutic dose.