Blood acidosis due to anaerobic metabolism is frequent in critically ill patients. To evaluate the changes in BE, AG and pH in case of hyperlactatemia, an cross-sectional study was conducted in 120 patients undergoing open heart operation in which acid-base status, electrolytes and serum lactate levels determined during and after operation. The results revealed that BE decreased but AG unchanged in case of serum hyperlactatemia and that there was no or weak correlation between serum lactate levels and BE or AG, and marked hyperlactatemia in post-operative severe acidosis.