The gastric acid secretion during stimulation with histamine given via a continuous intravenous infusion (i.e. during steady state conditions) was studied in 13 patients with prepyloric ulcers and in 21 patients with combined gastric and duodenal ulcers. The rate of volume secretion, the acidity and the acid output were analysed separately. The results were compared with earlier studies of patients with gastric ulcer and duodenal ulcer. Starting from the secretory pattern these four groups of ulcer were characterized. Gastric ulcers thus show hypoacidity. The pyloric ulcers do not differ from a normal material. The combined gastric and duodenal ulcers and the duodenal ulcers show hypersecretion, most pronounced in duodenal ulcers.