A calorimeter suitable for measuring human energy expenditure has been assembled by the US Department of Agriculture in Beltsville, Maryland. The room-sized calorimeter is 3.05 X 2.74 X 2.44 m (20.39 m3). Direct and indirect calorimetry methods are used to simultaneously measure heat emission and energy expenditure. A water-cooled gradient layer chamber is used to measure human heat production directly. Indirect calorimetry is ascertained by measuring the changes in gas composition of the air entering and existing the chamber. The inlet and outlet air are each sampled three times every 100 s with a multiple gas analyzer to determine carbon dioxide and methane production and oxygen consumption within the chamber. A total of 30 measurements, which include temperature, pressure, gas fractions, flow rates, direct heat transfer, electrical power, and motion within the chamber, are converted from electrical to digital signals and recorded on magnetic disk nine times each minute. Real-time calculations for directly and indirectly measured energy expenditure are continuously updated and displayed. The performance of the calorimeter is assessed by the combustion of alcohol within the chamber. Results of these tests indicate that the direct heat recovery is 99.7 +/- 2.6 (SD) % and the indirect heat recovery is 100.3 +/- 1.3 (SD) %.