In contrast to the steady low intragastric pH, the pH in the proximal duodenum shows wide, rapid, and frequent fluctuations. This change in pH pattern may be used to localize the pylorus and thereby ensure reproducible measurements of duodenal pH at a known and reproducible distance from the pylorus. To validate this method of localizing the pylorus, simultaneous measurements were performed of the transpyloric pH and potential difference (PD) profile and of the pH and pressure profile in 10 normal subjects. pH-metry and PD-metry localized the pylorus within the same 1.5 cm in 82% of 104 5-min periods, and pH-metry and manometry localized the pylorus within the same 5 cm in 72% of 77 10-min periods. This agreement and accuracy seem satisfactory for most studies of intraluminal pH in the duodenum and make reliable long-term ambulatory recording of duodenal pH possible without serial roentgenogram controls.