As part of a joint NASA-ESA-DARA study on the effects of chronically increased CO2 concentrations in ambient air, changes in parameters indicating aerobic capacity were investigated by cycle ergometry. Two potential sources for reductions of aerobic capacities were hypothesized: 1) the adaptations to CO2 such as reduction in H(+)-buffer capacities which may influence muscle metabolism; 2) the reduced physical activities which may lead to a detraining effect. Four subjects were exposed to 0.7% and 1.2% CO2 concentration in a confined compartment for 23 d each with 3 mo in between the two campaigns. A combined exercise test was applied before, during (on days 5, 11, and 22) and after CO2 exposure. Comparing steady-states at 30 W and 80 W power, elevated ventilation was found increased during CO2 exposure with significant differences between the two campaigns. Peak oxygen uptake decreased over the period of CO2 exposure, but was found not significantly different on day 5 compared to pre-exposure measurements. This decrease was not dependent on the CO2 concentration. The lactate concentration at low exercise intensities was found elevated during CO2 exposure. A shift in reverse direction was observed after the CO2 exposure. Since peak oxygen uptake did not differ on day 5 and the lactate concentration was found increased, it was concluded that the potential changes in muscle metabolism by adaptation to elevated CO2 concentrations did not influence the aerobic capacities. Therefore, it was concluded that the changes in aerobic capacities are the result of the reduced physical activities of the subjects while living in the confined compartment.