These studies were designed to determine whether the intestinal phase of gastric acid secretion is accompanied by an increase in serum gastrin concentration. Seven dogs with a gastric fistula and a duodenal fistula prepared so as to allow perfusion into the duodenum without reflux into the stomach were used. Gastric secretion and blood samples for serum gastrin determination were collected during intestinal perfusion with either 0.15 M NaCl or 5% liver extract at 200 ml hr(-1) for 2 hr. The mean +/- SE gastric acid secretory response during the second hour of perfusion with 0.15 M NaCl was 3.1 +/- 1.0% of the maximal acid output (MAO) to histamine, significantly (P less than 0.05) greater than the control value with no intestinal perfusion (0.18 +/- 0.07%). The mean +/- SE response during the second hour of perfusion with 5% liver extract was 7.4 +/- 2.2% of MAO, significantly (P less than 0.05) greater than both control and NaCl perfusion values. There was no significant change in peripheral serum gastrin concentration during the response to perfusion with either NaCl or liver extract. We conclude that, under the conditions of the present study, the intestinal phase of gastric acid secretion was not accompanied by an increase in serum gastrin concentration.