The purpose of this study was to examine whether excess CO_2 output during and after short duration-high intensity exercise would be a valid indicator of bicarbonate buffering action and to investigate its relationship with exercise performance. Seven competitive university sprinters performed 40-s maximal cycle ergometer sprinting. Excess CO_2 output during and after exercise was obtained through respiratory gas analysis. Furthermore, lactate concentration, pH, bicarbonate ion concentration ([HCO_3^-]) and CO_2 partial pressure were obtained through arterial blood sampling. Excess CO_2 output per unit time (Excess Vco_2) was calculated by subtracting Vco_2 from VO_2 during and after exercise. Peak power and mean power were measured as an index of exercise performance. The process of the ExcessCO_2 accumulation was fitted to the process of the [HCO_3^-] decrease during and after exercise. ExcessCO_2, calculated as the sum of Excess Vco_2 from the start of exercise until 10 mm after exercise, was significantly correlated with [HCO_3^-] decrease (r = 0.904, p<0.0l). A significant correlation was also demonstrated between Excess CO_2 and mean power during the 40-s maximal cycle ergometer sprinting (r = 0.859, p<0.0l). These results suggest that Excess CO_2 during and after short duration-high intensity exercise is a valid indicator of bicarbonate buffering action, and that it could be an important factor in determining exercise performance.