Gastric intramucosal pH (pHi) was examined to estimate the splanchnic circulation after open-heart surgery in fourteen patients without low cardiac output syndrome including five patients with coronary artery bypass grafting, five with mitral valve replacement, two with aortic valve replacement and two mitral and aortic valve replacement. A gastric tonometric catheter was inserted after surgery and pHi was measured immediately, 6, 12 and 24 hours after the admission to the intensive care unit (ICU). On admission, the cardiac index was over 4l·min-1·m-2 and mixed venous oxygen saturation over 70%. However, pHi was lower than 7.32 of the critical level and it recovered to the normal level within 24 hours after the admission to the ICU. These data indicate that the splanchnic blood flow decreased after cardiopulmonary bypass although cardiac output was preserved in normal range. The monitoring of pHi is noninvasive and useful to estimate the splanchnic circulation.