This investigation evaluated the measurement reliability and reactivity of repeated measurements of resting energy expenditure using different methods of data collection. Thirty subjects were randomly assigned to either face mask, mouthpiece/nose clip, or ventilated canopy collection systems for 45 min a day over 3 days. Results indicated that there were no significant differences among data collection systems nor were there any differences either within or between systems over time. Across all three days of measurement, the first 5 min of data collection on all systems were reactive compared to the subsequent 40 min of assessment. Analyses indicated that acceptable reliability coefficients were obtained after 20 min of continuous data collection in the ventilated canopy and after 40 min with the face mask after a 5-min acclimation period. The results suggest that, following an acclimation period, a single 20-min assessment of resting energy expenditure in both a face mask and ventilated canopy are stable and reliable.