We performed simultaneous, ambulatory monitoring of luminal pH in the stomach and in the duodenum in 20 healthy volunteers. The objectives were to determine normal pH ranges and patterns and the influence of food and posture on pH, and to study spontaneous alkaline shifts in the fasting stomach. Antimony electrodes were placed in the antrum and duodenum (second portion) under fluoroscopy. The pH values were recorded every 4 s and the data were analyzed by computer. During daytime and nighttime, when fasting, the gastric pH was generally less than pH 2. Eating increased the peak gastric pH to 4.1 ± 0.2. Alkaline episodes (pH ≧ 4 for 60 s or more) occurred in all subjects frequently in the early morning hours; the mean frequency was 13.3 ± 2.7 per 24-hour period. Duodenal pH was generally neutral (pH > 6 for 66% of the study period). There were 156 ± 20 acid spikes (pH < 4, duration 15 s) per 24 h. Results indicate that long-term, ambulatory pH-metry in stomach and duodenum is possible, that eating and posture do influence pH profiles and that spontaneous alkaline pH shifts regularly occur in healthy individuals.