The aim of this study was to investigate the acid-base balance during repeated cycling sprints in children and adults. Eleven boys (9.6 ± 0.7 yr) and ten men (20.4 ± 0.8 yr) performed ten 10-s sprints on a cycle ergometer separated by 30-s passive recovery intervals. To measure the time course of lactate ([La]), hydrogen ions ([H⁺]), bicarbonate ions ([HCO]), and base excess concentrations and the arterial partial pressure of CO₂, capillary blood samples were collected at rest and after each sprint. Ventilation and CO₂ output were continuously measured. After the 10th sprint, concentrations of boys vs. men were as follows: [La], 8.5 ± 2.1 vs. 15.4 ± 2.0 mmol/l; [H⁺], 43.8 ± 1.3 vs. 66.9 ± 9.9 nmol/l (P < 0.001). Significant correlations showed that, for a given [La], [H⁺] was lower in the boys compared with the men (P < 0.001). Significant relationships also indicated that, for a given [La], [HCO] and base excess concentration were similar in the boys compared with the men. Moreover, significant relationships revealed that, for a given [H⁺] or [HCO], arterial partial pressure of CO₂ was lower in the boys compared with the men (P < 0.001). The ventilation-to-CO₂ output ratio was higher in the boys during the first five rest intervals and was then higher in the men during the last five sprints. To conclude, during repeated sprints, the ventilatory regulation related to the change in acid-base balance induced by lactic acidosis was more important during the first rest intervals in the boys compared with the men.