The purpose of this study was to elucidate the effect of lung-bag volume shrinkage on P'VCO2 estimated by the CO2 rebreathing method and to correct its effect in calculation of cardiac output. The P'VCO2 was extrapolated with the linear relationship between dPACO2/dt and PACO2 during rebreathing. The P'VCO2 was higher than real mixed PCO2 by 0.82 mmHg at rest, by 1.35 mmHg at 50W, 1.57 mmHg at 100W, and 3.06 mmHg at 150W considering the effect of lung-bag volume shrinkage detected by argon concentration. The cardiac output obtained by the CO2 rebreathing method with correction of above differences was in good agreement with that determined by the acetylene method (r = 0.930, p less than 0.001). When the cardiac output was plotted against O2 uptake, our results and the regression line were fairly close to the reported regression lines.