To determine the influence of changes in gastric juice pH due to intravenous administration of pentagastrin and omeprazole on intramucosal regional Pco ₂ (Pr_CO2 ), we investigated 17 healthy human volunteers. Gastric juice pH was obtained from a glass pH electrode for continuous gastric juice pH measurement and Pr_CO2 was measured by using automated air tonometry. After baseline (8:00 a.m.–9:00 a.m.) the subjects received 0.6 μ g/kg/h pentagastrin intravenously for 1 h (9:00 a.m.–10:00 a.m., after stimulation 10:00 a.m.–11:00 a.m.) and 40 mg omeprazole intravenously (after omeprazole 11:00 a.m.–3:00 p.m.). Following pentagastrin administration gastric juice pH significantly decreased from 1.2 ± 0.4 to 0.6 ± 0.4 (mean ± SD, p < 0.007, versus baseline), whereas omeprazole transiently increased luminal pH up to 4.4 ± 1.7 (p < 0.007 versus baseline). These subsequent changes in gastric juice pH were accompanied by a significant increase in Pr_CO2 from 48 ± 12 to 61 ± 17 mm Hg (p < 0.007 versus baseline) and a decrease to 44 ± 5 mm Hg (p < 0.002 versus pentagastrin), respectively. A gastric juice pH > 4 considerably reduces mean gastric Pr_CO2 and interindividual variability. Thus omeprazole may improve the validity of gastric tonometry data.