Tissue oxygenation in the gastrointestinal tract was studied in a porcine model in which septic shock was induced by fecal peritonitis. The oxygen delivered was estimated by measuring the portal venous blood flow and the calculated arterial oxygen saturation. The oxygen consumption of the gut, including the pancreas and spleen, was monitored by measuring the portal venous blood flow and the difference between the calculated arterial oxygen and the measured portal venous oxygen saturation. In addition, the oxygenation of the gut mucosa was followed via the tonometric technique. Furthermore, lactate was measured in arterial and portal blood. The experimental animals were divided into two groups, one control (n = 6) and one experimental (n = 6). Peritonitis was introduced by installation of a standardized amount of autologous feces into the abdominal cavity. The animals were followed for 5 hr. Very early during the course of sepsis there was a fall in gut intramucosal pH (pHi), and this was evident before any reduction in splanchnic DO2. Furthermore, an early increase in splanchnic VO2 was evident simultaneously with the fall in pHi. Arterial pH and lactate were not able to detect the inadequate regional tissue oxygenation. It is concluded that pHi measured with the tonometric technique is sensitive in detecting gut mucosal ischemia, and it is therefore highly likely that tonometry would be a valuable method in monitoring severe ill patients.