Possibility of applying the rebreathing method for estimating the cardiac output in case when ventilation did not correlate with the lung blood flow is discussed. pCO2 of mixed venous blood was determined by its equilibration with pCO2 of the alveolar air by the rebreathing method. Gas mixture containing almost as much CO2 as in venous blood was created by the patient by rebreathing into a bag, 2--3 litres in volume, filled with oxygen. By the differences in the shape of the capnogram the ununiformity in the distribution of ventilation in respect to the cardiac output was judged. In the absence of uniformity pCO2 of the arterial blood was measured. This correction allows evaluation of cardiac output in patients with cardiac and lung diseases. A possibility of using standard nomograms for evaluating CO2 store in the arterial and venous blood in case of serious disturbances of the acid-base balance is discussed.