In patients with peptic ulceration, both vagal stimulation by insulin hypoglycaemia and stimulation by pentagastrin cause pepsin 1 to be secreted into gastric juice. There is a secretory threshold for pepsin 1, below which only pepsins 3 and 5 are secreted. Pepsin 1 accounts for an increasing proportion of the total peptic activity/ml of gastric juice as the total activity increases. Higher concentrations of pepsin 1 in the basal gastric secretion occurred significantly more frequently in patients with duodenal ulcer than with gastric ulcer. In these patients there may be an increased 'background' secretory drive.