OBJECTIVE The aim of the study was to assess gastric mucosal pH during certain parts of a major cardiac procedure (hypo- and normothermic), and then throughout the first postoperative day. METHODS Gastric mucosal pH was measured using a tonometer, in 15 patients subjected to elective CABG procedures. The patients were also assessed haemodynamically and serum lactate concentrations were measured. RESULTS It was found that cardiopulmonary bypass did not suppress visceral perfusion. The most critical was the period between 4th and 12th postbypass hour when a tendency towards decreased cardiac output and oxygen delivery was noted. At the same time oxygen consumption was increasing which resulted in triggering of anaerobic metabolic pathways, that was mirrored by significant rise in serum lactate levels. During this period of time the lowest, although not critical levels of gastric mucosal pH were recorded, suggesting a relative decrease in splanchnic perfusion. CONCLUSIONS Extracorporeal circulation does not significantly compromise splanchnic perfusion. Tonometry is a valuable, non-invasive method of visceral oxygenation monitoring. The first postoperative day is an especially critical time for a cardiac patient--a frequent control of haemodynamic and acid-base balance parameters is absolutely mandatory.