We investigated the relation between age and respiratory control during exercise in 80 children aged 6.4-17.6 y (42 male, age, 11.6 ± 3.3 y; body weight, 41.3 ± 14.7 kg; and 38 female, age, 12.6 ± 2.7 y; body weight, 42.5 ± 9.3 kg). Exercise tests were performed on a treadmill after a standard symptom-limited Bruce protocol. Data were analyzed separately for boys and girls. At rest and at peak exercise, the ratio of minute ventilation to carbon dioxide production(˙VE/˙VCO2), the ratio of effective alveolar ventilation to carbon dioxide production(˙VA/˙VCO2), the ratio of pulmonary dead space to tidal volume (Vd/Vt), and arterial PCO2(PaCO2) were not correlated with age. At the exercise intensity of ventilatory anaerobic threshold, the PaCO2increased, and the˙VA/˙VCO2decreased, significantly with age, whereas theVd/Vtwas not related to age. Linear regression analysis demonstrated that the slope of the relationship between ˙VE and˙VCO2(Δ˙VE/Δ˙VCO2) and the slope of the relationship between˙VA and˙VCO2(Δ˙VA/Δ˙VCO2) decreased with age. There were no gender differences in the regression slopes and the intercepts of the relationships. Results show that younger children breathe more during exercise to eliminate a given amount of CO2to keep PaCO2set point slightly but significantly lower than older children. This age dependence must be considered in investigating ventilatory control during exercise in children.Abbreviations:˙VE,minute ventilation;˙VA,effective alveolar ventilation;Vt,tidal volume;Vd,pulmonary dead space;Vd/Vt, ratio of pulmonary dead space to tidal volume;˙VCO2,carbon dioxide production;PaCO2,arterial PCO2;PETCO2,end-tidal PCO2;Δ˙VE/Δ˙VCO2,slope of the relationship between˙Ve and˙VCO2;Δ˙VA/Δ˙VCO2,slope of the relationship between˙VA and˙VCO2;VAT,ventilatory anaerobic threshold;RR,respiratory rate;STPD,standard pressure(and) pressure, dry