There is a lack of information about the effect of omeprazole or other antisecretory drugs on intraduodenal pH. Aim of the study was to document the variation over time of intraduodenal pH during a 24-hr period and to simultaneously study the effect of omeprazole 40 mg once daily on intragastric and intraduodenal pH in healthy H. pylori-negative subjects. In a randomized, placebo-controlled study, eight subjects (five women, three men, mean age 22.7 years) received oral 40 mg omeprazole or placebo once daily for eight days. On day 7, intragastric and intraduodenal pH was measured continuously for 24 hr, using two miniature glass-membrane electrodes placed in the stomach (fundus) and in the distal third of the duodenum. The 24-hr median intraduodenal pH was 5.95 with placebo and 5.85 with omeprazole. Median intragastric pH was 1.68 without and 4.93 with omeprazole. During omeprazole therapy, intragastric pH fell below 4.0 in five of eight subjects. In the 2- and 3-hr postprandial periods, the percentage of time with pH < 5 was significantly reduced with omeprazole. In healthy subjects, 24-hr median and postprandial pH in the distal part of the duodenum was lower than previously thought. Omeprazole significantly reduced the percentage of time with pH < 5 postprandially. At night, intragastric pH fell below 4.0 with omeprazole 40 mg once daily. Omeprazole does not change 24-hr median intraduodenal pH significantly.