1. On the basis of previously described dose-response curves, small doses of pentagastrin were infused in duodenal ulcer patients before and after vagotomy. No increase or decrease in gastric acid output was noted, confirming that the doses of pentagastrin used were sub-threshold.

2. In twenty-six duodenal ulcer patients the effect of sub-threshold doses of pentagastrin on the insulin response resulted in three distinct patterns. (i) In subjects with clearly functioning vagal pathways, no increase in acid output occurred after insulin when a sub-threshold dose of pentagastrin was added; (ii) patients with equivocal acid responses to insulin alone showed augmentation of acid output when given insulin and a sub-threshold pentagastrin infusion; (iii) patients with no response to insulin after truncal vagotomy showed some increase in acid output to the combined agents, and in two patients, criteria for a clearly positive acid response were satisfied.

3. We conclude from these studies that potentiation exists between insulin-induced cholinergic stimulation and infusion of sub-threshold doses of pentagastrin in man.

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