Two portable metabolic cart systems of indirect calorimetry (Deltatrac Metabolic Monitor, 2900 MMC System) were validated. CO2 and N2 were delivered at precise rates into a constructed lung model to simulate CO2 production (VCO2) and O2 consumption (VO2). VCO2 (200-400 ml/min) and VO2 (250-750 ml/min) were measured at varying combinations of minute ventilation (VE) (6, 10, 20 liter/min) and FiO2 (0.21, 0.30, 0.60, 0.80). VCO2 was measured with overall errors of 1.5% and 2.4% for the Deltatrac and 2900 monitors, respectively. VO2 was measured with overall errors of 1.9% and 3.2% by the Deltatrac and 2900 monitors, respectively. Both monitors performed equally well for measurement of VO2 at FiO2 up to 0.6, but the Deltatrac had less error for measurements of VO2 at FiO2 of 0.8.