Serum gastrin concentrations and gastric acid secretion were measured during intravenous infusion of gastrin heptadecapeptide (G-17) (0, 7, 22.1, 70, 221, and 700 pmol/kg X h) in 15 duodenal ulcer patients and 15 healthy controls. Ulcer patients developed higher serum gastrin concentrations during G-17 infusion due to nearly twofold slower clearance of gastrin (8.8 vs. 15.7 ml/kg X min; P less than 0.01). Despite slower clearance of G-17, ulcer patients had plasma elimination half-times for G-17 similar to controls (6.0 vs. 6.1 min, respectively). Thus, calculated volume of distribution for G-17 was lower in ulcer patients than controls (78.5 vs. 140.7 ml/kg; P less than 0.025). For any serum gastrin during gastrin-17 infusion, acid secretion (millimoles per hour) was higher in ulcer patients than in controls. However, when acid secretion was expressed as a percentage of peak acid output to G-17 (to correct for differences in parietal cell mass), curves relating acid secretion to serum gastrin were identical in ulcer patients and controls.