The effect of intrajejunal infusion of a fat solution on pentagastrin-stimulated (0.5 microgram/kg-hr) gastric acid secretion was studied in five duodenal ulcer patients before and three to five months after truncal vagotomy. Preoperatively mean acid output was reduced from 41 +/- 5.2 meq H+/hr to 15.4 +/- 1.3 meq H+/hr (p less than 0.005), and postoperatively from 17.4 +/- 2.6 meq H+/hr to 12.7 +/- 2.4 meq H+/HR (P less than 0.01), demonstrating that fat induced jejunal inhibition of gastric acid secretion partly or wholly is humorally mediated. Furthermore the study shows that fat induced jejunal inhibition of acid secretion in duodenal ulcer patients seem to be of the same magnitude as previously found in healthy subjects.