Six obese patients undergoing end-to-side jejunoileal bypass had gastric acid output and serum gastrin measurements performed preoperatively and within three months postoperatively. Meat extract stimulated acid output was higher in obese than in normal patients. Following the bypass procedure, basal and meat extract stimulated acid output significantly decreased. Basal and meat extract stimulated serum gastrin levels did not change following bypass. These studies demonstrate that after a small intestinal bypass, acid output decreases. Gastrin plays no role in this change.