The purpose of this study was to compare oxygen uptake (\(\dot V{\text{O}}_{\text{2}} \)) values collected with a new portable indirect calorimeter (AeroSport TEEM 100 Metabolic Analysis System) against a more traditional large calorimeter system that has been reported to be valid and reliable (SensorMedics 2900 Metabolic Measurement Cart). Minute ventilations ranging from rest up to heavy exercise were compared with simultaneous measurements from a 120-1 Tissot gasometer. Each of the three TEEM 100 pneumotachs were tested. Three hundred and sixty-one separate ventilation tests were performed using the low-flow, medium-flow, and high-flow heads of the portable calorimeter. For each of the pneumotachs, the correlation between the portable calorimeter values and the gasometer values exceededr = 0.94. The standard error of estimate for the low-, medium- and high-flow pneumotach were 5.96, 4.89 and 9.0%, respectively, expressed relative to the mean gasometer value. Simultaneous measurements of\(\dot V{\text{O}}_{\text{2}} \) using the portable calorimeter and the SensorMedics 2900 unit were compared during rest and at work rates starting at zero watts, increasing by 25 W to 150 W. Each work rate was of 4 min duration. The average of data from minutes 3 and 4 were used in all analyses. There was very close agreement between the two metabolic measurement systems. Except at the 100-W work rate, where the\(\dot V{\text{O}}_{\text{2}} \) difference was small (3.9%), yet statistically significant, all of the other differences in\(\dot V{\text{O}}_{\text{2}} \) were small and non-significant. The scatter plot of\(\dot V{\text{O}}_{\text{2}} \) for the SensorMedics versus the portable Aero-Sport calorimeter revealed close agreement; the correlation wasr = 0.96, (SEE = 3.95%). It was concluded that the AeroSport TEEM 100 portable calorimeter system produces valid data at rest and at low to moderate work rates compared to a criterion, large system.