The effect of somatostatin on pentagastrin-induced gastric-acid secretion was measured in three healthy volunteers and four patients with duodenal ulcer. Somatostatin (100, 200 and 400 mug/h) inhibited secretory volume and acid concentration simulated by pentagastrin infusion (2 mug/kg - h), the effect being dose-dependent. In the patients with duodenal ulcer somatostatin given as a bolus (50 mug) and by subsequent infusion (150 mug/h) reduced maximal acid output after subcutaneous injection of 2.0 mug/kg pentagastrin by 68-96%.