AbstractThe influence of rebreathing time when using a CO2rebreathing method for graphical determination of the mixed venous CO2tension was studied. An almost significant difference of 0.6 mm was found between values obtained at rebreathing rates of 30 and 50 breaths per min with rebreathing times of 22 and 13 sec. This should give a difference at rest of about 5 per cent when the cardiac output is calculated, with the higher value when the fast rate is used for the mixed venous CO2determination. The solubility of CO2in lung tissue probably doses not cause this difference.