A monitor was developed to track weightlessness deconditioning aboard the National Aeronautics and Space Administration (NASA) Space Station by measuring the O2 uptake (VO2) and CO2 production (VCO2) and calculating maximum VO2 and anaerobic threshold during an exercise stress test. The system uses two flowmeters in series to achieve a completely automatic flow calibration, and it uses breath-by-breath compensation for sample line transport delay. The accuracy of the system was measured over the range of VO2 and VCO2 from 100 to 800 ml/min by means of simulation. Accuracy was 0.54% for VO2 and 2.9% for VCO2. The system was further evaluated using two laboratory methods, the first method being comparison with a breath-by-breath system. As volunteers performed a maximum effort on a cycle ergometer, the mean difference in readings between the two systems was 17 ml/min for VO2 and 8.0 ml/min for VCO2. The correlation coefficient squared was greater than 0.96 for both. The second laboratory test was to use the system for 2 mo in a Human Performance Laboratory. Readings of maximum VO2 (VO2max) and anaerobic threshold were repeatable and consistent with the individual's activity level. The accuracy and convenience of operation will make this a valuable instrument aboard the Space Station.