1. The acid secretory responses to insulin of forty-seven duodenal ulcer patients after truncal vagotomy and drainage were classified according to the criteria of Hollander (1946) into positive or negative.
2. The acid response to ranges of small doses of pentagastrin was studied in these two groups. In the group with a positive acid response after insulin the dose-response pattern to pentagastrin was very similar to that in preoperative patients.
3. In those who failed to satisfy criteria for a positive response after insulin the dose-response curve to pentagastrin appeared to shift to the right. It had a lower level and a smaller slope. Truncal vagotomy appeared to cause an eightfold increase in the threshold dose found in preoperative patients.
4. Supersensitivity of the stomach to small doses of pentagastrin after vagotomy was not apparent in the present study.