BACKGROUND The present study was designed to investigate whether omeprazole changes the characteristics and thereby the functions ascribed to fasting intestinal motility, postprandial motility, postprandial pH, and gastric emptying. METHODS Ten healthy subjects were investigated. The studies were performed after 10 days of treatment with 40 mg omeprazole daily/placebo. Duodenal pressures and intragastric pH were detected by strain-gauge transducers and a pH electrode attached to a miniature computer. The meal consisted of an omelette labelled with 99mTc-sulphur colloids followed by 150 ml water labelled with 111In-diethylenetriamine pentaacetic acid. RESULTS The difference in fasting intragastric pH between the two series was highly significant. The profile from the placebo series showed a relationship between phase activity and pH. The pH increased from phase I (median, 1.3; 95% confidence interval (CI), 0.9-1.6) to a maximum at 25% (1.8 (0.9-2.1)) and 50% (1.6 (1.1-3.8)) of cycle duration and decreased thereafter until the end of the cycle. The profile from the omeprazole series showed significantly higher values during the entire cycle but no relationship between phase activity and pH. Pretreatment with omeprazole was followed by a delay in gastric emptying of liquid at 30 min (64% (49%-66%) (omeprazole series) versus 78% (67%-83%); P < 0.01) and solid at 180 min (71% (48%-86%) (omeprazole series) versus 96% (87%-100%); P < 0.01). There was no significant difference in duration of postprandial motility (305 min (157-350 min) (omeprazole) versus 259 min (129-403 min)). CONCLUSIONS Omeprazole eliminates the temporal relationship between intragastric pH and characteristics of the migrating motor complex and induces a delay in gastric emptying of both liquid and solid. A non-significant increase in duration of postprandial motility may represent a type-II error.