The measurement of whole body heat loss in humans and the performance characteristics of a modified Snellen whole body air calorimeter are described. Modifications included the location of the calorimeter in a pressurized room, control of operating temperature over a range of - 15 to + 35 degrees C, control of ambient relative humidity over a range of 20-65%, incorporation of an air mass flow measuring system to provide real time measurement of air mass flow through the calorimeter, incorporation of a constant load 'eddy current' resistance ergometer and an open circuit, expired gas analysis calorimetry system. The performance of the calorimeter is a function of the sensitivity, precision, accuracy and response time characteristics of the fundamental measurement systems including: air mass flow; thermometry and hygrometry. Calibration experiments included a calibration of the air mass flow sensor, the response of the thermometric measurement system for dry heat loss and the response of the hygrometric measurement system for evaporative heat loss. The air mass flow system was evaluated using standard differential temperature procedures to demonstrate linearity and sensitivity of the device. A novel procedure based on differential hygrometry was developed to ascertain the absolute calibration of air mass flow by resolving the unique system coefficient K. The results of the hygrometric calibration demonstrate the air mass flow response of the system is linear over the range of air mass flows from 6 to 15 kg min(-1). R(2) was 0.995. The average half response time (tR50) was 14.5 +/- 2.1 s. Similarly the results of the thermometric calibration demonstrate that the response of the apparatus is linear over the range of power input measured (coefficient of linearity R(2)=0.9997) with a precision of 0.72 W and an accuracy to within 0.36 W. The average (tR50) over all conditions was 6.0 +/- 1.9 min. In summary, modifications brought to the Snellen calorimeter have significantly improved the precision, accuracy and response time characteristics of the previous system while extending its operating range.