To determine the relationship between gastric intramucosal pH and several other indices of splanchnic perfusion in patients undergoing cardiopulmonary bypass. Prospective, single-arm study. University Hospital. Elective cardiac surgery patients (n = 8), free of hepatic disease, were studied. Before anesthetic induction, a triple-lumen, heparin-bonded fiberoptic catheter was inserted into the hepatic vein under fluoroscopic guidance. An identical catheter was inserted into the pulmonary artery. After endotracheal intubation, a nasogastric tube modified to permit measurement of gastric intramucosal pH was inserted into the stomach. Systemic oxygen delivery (Ḋo2), and arterial, mixed venous, hepatic venous, and femoral venous blood gases and lactate concentrations were recorded at the following times: immediately before induction of anesthesia (time 1); during atrial cannulation (time 2); after 30 mins of hypothermic cardiopulmonary bypass (time 3); 15 mins after termination of cardiopulmonary bypass (time 4); and 1 hr after arrival in the ICU (time 5). Hepatic venous hemoglobin saturation (So2) and mixed venous hemoglobin saturation (SJOURNAL/ccme/04.02/00003246-199110000-00009/ENTITY_OV0456/v/2017-07-20T220804Z/r/image-pngo2) were monitored continuously from times 1 to 5. Gastric intramucosal pH was recorded at times 2, 3, 4, and 5. The hepatic catheter was removed as soon as the last samples were collected in the ICU. The square of the weighted mean correlation coefficients (rw)2 for gastric intramucosal pH vs. hepatic venous lactate concentrations, gastric intramucosal pH vs. hepatic venous Po2, and gastric intramucosal pH vs. hepatic venous pH were (rw)2 = .50, (rw)2 = .58, and (rw)2 = .32, respectively. Systemic Ḋo2, hepatic venous lactate concentrations, hepatic venous Po2, and hepatic venous pH were significant determinants in the multiple regression model for gastric intramucosal pH (r2 = .89). There were significant differences between SJOURNAL/ccme/04.02/00003246-199110000-00009/ENTITY_OV0456/v/2017-07-20T220804Z/r/image-pngo2 and hepatic venous So2 at times 4 and 5. Gastric intramucosal pH may provide a minimally invasive way to monitor the adequacy of splanchnic Ḋo2 in patients undergoing cardiopulmonary bypass. Additional data are necessary to determine whether low gastric intramucosal pH is truly a marker of supply-dependent oxygen uptake across the hepatosplanchnic vascular bed under these conditions.