The purpose of the present study was to obtain the immediate CO₂ storage capacity at the onset of exercise. The CO₂ stores at the onset of the exercise were calculated from the difference between the respiratory gas exchange ratio (R) and the metabolic gas exchange ratio (RQ; R obtained at 5.5min of exercise). The CO₂ stores per body weight (CO₂ stores/w) were linearly related to the CO₂ pressure (P′^V_CO2) determined by the CO₂ rebreathing method (r=0.713, p<0.001), the slope being 0.330ml/(mmHg•kg). The CO₂ stores were then corrected for change in O2 stores with exercise, that defined as total CO₂ stores. P′VCO₂ was also corrected for the effect of lung-bag volume shrinkage and Haldane effect during CO₂ rebreathing, that defined as true PVCO₂. The total CO₂ stores/w were also related linearly to the true P′^V_CO2 (r=0.725, p<0.001), the slope of the regression line defined as the immediate CO₂ storage capacity being 0.650ml/(mmHg•kg).