Mifentidine is a new H2-receptor antagonist with distinct characteristics of potency and long plasma half-life. The aim of this study was to evaluate the effects of mifentidine on peptone meal-stimulated gastric acid secretion. Nine duodenal ulcer patients in remission were enrolled in the study and given in double-blind and at random, on two different occasions, a single tablet of 10 or 20 mg mifentidine or placebo according to an incomplete balanced block design. Ninety min after ingestion of the drug, basal gastric secretion was collected for 30 min and volume, pH and acid output determined. Thereafter, the acid output following peptone meal-stimulation was measured for 2 h by a modified version of the intragastric titration method of Thompson and Swierczek. Plasma samples were collected for gastrin and mifentidine determinations. Basal acid output was strongly inhibited by both the low dose (-78%) and the high dose (-98%) (p less than 0.01). The peptone meal-stimulated acid output was reduced in a dose-dependent manner (-45% by 10 mg and -90% by 20 mg). The drug did not affect the fasting serum gastrin levels but increased, although not significantly, the gastrin response to food. The log of the area under the mifentidine plasma levels correlated linearly with total acid output (p less than 0.01). The results of this study indicate that mifentidine dose-dependently suppresses basal acid secretion and reduces peptone-stimulated gastric acid secretion in duodenal ulcer patients.