To test the hypothesis that duodenal ulcer is caused by abnormal acidity, it is necessary to measure acidity in the ulcer-bearing part of the duodenum. Previous investigators have attempted to do this, either by aspirating duodenal contents (Morton, 1929; Berk, Rehfuss, and Thomas 1942a; 1942b; Atkinson and Henley, 1955) or by measuring pH in situ with a glass electrode (Eyerley, 1940; Tomenius and Williams, 1960; Rovelstad and Maher, 1962; Bircher, Mann, Carlson, Code, and Rovelstad, 1965). The results in general are not very satisfactory because it is difficult to keep a tube in the short duodenal bulb. Using two glass electrodes we have been able to control the position of the tube continuously and to measure pH in the duodenal bulb for long periods.