Gastric and duodenal bulb pH was measured simultaneously with a multiple pH electrode system in 15 duodenal ulcer patients. A single dose of 10 ml antacid was given 120 min after a liquid standard meal, and the pH was measured for another 90 min. The effect of antacid on duodenal pH, expressed as the time pH is kept above 3.0, was on an average 60 min--shorter in patients with a high gastric acid secretion and longer in normosecretors (p less than 0.01). Regression analysis of simultaneously measured gastric and duodenal bulb pH after antacid showed a highly significant linear correlation with a slope of 1.43, which is significantly greater than 1.0 (p less than 0.01), indicating that antacid has a more pronounced effect on duodenal pH than on gastric pH.