In 8 outpatients with duodenal ulcer the effect of a single dose of 50 mg pirenzepine is compared to a 3-7 days treatment with 2x50 mg. Basal and peptone-stimulated acid output is measured. A single dose of 50 mg pirenzepine reduced basal acid output by 39,9% and the stimulated acid secretion by 21,3%. Treatment for 3-7 days with 2x50 mg pirenzepine is significantly more effective. Basal acid output is reduced by 75,1% and stimulated acid output by 54,0%. No acid inhibition can be demonstrated 12 hours after the last drug administration. Basal serum gastrin levels are little but significantly increased after chronic treatment.