Oxygen consumption (VO2) and carbon dioxide production (VCO2) rates were measured by electronically recording heart rate (HR) and physical activity (PA). Mean daily VO2 and VCO2 measurements by HR and PA were validated in adults (n = 10 women and 10 men) with room calorimeters. Thirteen linear and nonlinear functions of HR alone and HR combined with PA were tested as models of 24-h VO2 and VCO2. Mean sleep VO2 and VCO2 were similar to basal metabolic rates and were accurately estimated from HR alone [respective mean errors were -0.2 +/- 0.8 (SD) and -0.4 +/- 0.6%]. The range of prediction errors for 24-h VO2 and VCO2 was smallest for a model that used PA to assign HR for each minute to separate active and inactive curves (VO2, -3.3 +/- 3.5%; VCO2, -4.6 +/- 3%). There were no significant correlations between VO2 or VCO2 errors and subject age, weight, fat mass, ration of daily to basal energy expenditure rate, or fitness. VO2, VCO2, and energy expenditure recorded for 3 free-living days were 5.6 +/- 0.9 ml.min-1.kg-1, 4.7 +/- 0.8 ml. min-1.kg-1, and 7.8 +/- 1.6 kJ/min, respectively. Combined HR and PA measured 24-h VO2 and VCO2 with a precision similar to alternative methods.