Intraluminal pCO/ was measured in the stomach, small intestine, and sigmoid colon of pigs using balloon catheters (Tonomitor). With the simultaneously determined arterial blood HCO3 concentration, the intramucosal pH (pHi) could be calculated using the Henderson-Hasselbalch equation. Cardiac output and portal venous flow were measured using Swan-Ganz catheters. A series of 35 normotensive pigs were studied to achieve normal values of cardiac output, gastrointestinal pHi, and blood flow in pigs. In another series of pigs (n = 12), hemorrhage was induced in two steps and followed by retransfusion. Five additional animals served as controls. Gastric, small intestinal, and colonic pHi decreased with increasing degree of hemorrhage but remained unchanged in the controls. Following severe hemorrhage, pHi fell below the normal range in all pigs but one. It recovered only partly following retransfusion. Histological examination of specimens obtained following retransfusion revealed normal mucosa in six of eight investigated animals and superficial mucosal injury in the remainder, indicating that abnormal pHi may be found for a period in the microscopically normal gastro-intestinal mucosa. Monitoring pHi in the stomach, small intestine, or colon using tonometers could be a useful technique to reveal insufficient mucosal blood flow in the alimentary canal.