1. The acid secretory responses to a range of small doses of pentagastrin in 0·15 m-NaCl have been studied in thirty-one preoperative duodenal ulcer subjects. Acid output increased significantly above basal values when a dose of 0·064 μg h−1 kg−1 was given.
2. Control observations in sixteen duodenal ulcer patients using the saline solvent alone at identical rates of infusion showed no significant increase in acid output.
3. From the dose-response curves sub-threshold and threshold doses of pentapeptide are suggested for duodenal ulcer patients before truncal vagotomy.
4. Considerable variation in acid response was noted between patients given the same body-weight dose of pentapeptide. The results suggest that a ‘twilight zone’ of stimulation exists between the dose of pentagastrin by which few patients are stimulated and the dose by which the majority are stimulated. This may reflect some variation in the sensitivity to stimulation by pentagastrin from one patient to another.