Basal acid output (BAO) and histamine-stimulated acid output (MAO) were studied in 11 fasting rhesus monkeys. Serum gastrin and gastric inhibitory polypeptide (GIP) were assayed in eight. Fifty percent distal small bowel resection was performed and the studies were repeated at 6 weeks and 6 months. Highly significant increases were seen in BAO (0.37 "/- 0.05 to 1.73 +/- 0.37 mEq. per hour; P less than 0.005) and MAO (1.7 +/- 0.22 to 3.21 +/- 0.45 mEq. per hour P less than 0.01) at 6 weeks. These levels persisted at 6 months. The ratio BAO:MAO increased throughout the experiment, the changes being significant at both time intervals. A significant rise was noted in the levels of both fasting serum gastrin (P less than 0.05) and GIP (P less than 0.01) during the early phase. By 6 months the levels of both hormones had fallen and were not significantly different from the control values. It was concluded that 50 percent small bowel resection causes a persistent increase in gastric acid secretion. No corresponding sustained change in serum gastrin or GIP occurs.