Patients with duodenal ulcers and Helicobacter pylori infection have elevated plasma gastrin concentrations which fall after suppression of the organism. This may be due to H. pylori elevating the pH of the antral mucous layer, therefore preventing luminal acid from inhibiting gastrin release. To test this idea, we measured the plasma gastrin concentrations under basal conditions and in response to 4% peptone when the gastric lumen was maintained at pH 2.5 and at pH 5.5 by gastric perfusion. We studied 11 duodenal ulcer patients before and after suppression of H. pylori. Gastrin concentrations were significantly higher before suppression of H. pylori than after treatment in all three states; basal gastrin (pmol/l) fell from 9.2 (3.7-23, median and range) to 5.1 (1.7-15) after treatment; from 11.3 (3.8-29) to 5.9 (5.7-6.1) at pH 2.5 and from 15.2 (3.9-32) to 7.15 (6.1-14) at pH 5.5. The ratio of peptone-stimulated gastrin at pH2.5/pH5.5 was similar before (0.8; 0.5-1.7) and after (0.8; 0.5-1.1) suppression of H. pylori. These results indicate that infection with H. pylori increases basal and peptone-stimulated plasma gastrin concentrations, and that this response is independent of luminal pH.