Effects on ovine gastric function of procedures that increase intestinal unsaturated fatty acid content are unknown, and the present aim was to compare the effects of duodenal unsaturated and saturated fatty acids on gastric secretion in conscious sheep. During the maximal gastric secretory response to a meal, 10 ml gallbladder bile alone or with myristic, palmitic, and stearic acids and oleic, linoleic, and linolenic acids were infused into the duodenum at a rate of 5 g fatty acid . h-1 for 1 h. Compared with control 154 mM NaCl (100%), acid output was reduced to 4-7% of control with infusion of oleic, linoleic, and linolenic acids and myristic acids plus bile. Reductions in acid secretion persisted for up to 5 h from the end of infusion. In contrast, the infusion of palmitic and stearic acids with bile caused mean maximal reductions in acid output, respectively, to only 64 and 55% of control, and levels returned to control within 1 h of the end of infusion. Bile infusion alone caused no reduction in acid secretion. Under the conditions used, C18 unsaturated fatty acids and myristic acid were potent inhibitors of ovine gastric acid secretion. The lesser effects of palmitic and stearic acids were probably related to their reduced solubility and absorption.