A rapidly responding gradient layer calorimetry system for experimental animals is described. Monitoring of radiant and evaporative components of heat loss is accomplished by separate heat exchange systems that employ thermoelectric heat flowmeters.Calibration and test results with the equipment show the following: The response to energy input is linear in both the radiant heat exchange system (radiant calorimeter thermopile) and the circulating air heat exchange system (ventilatory calorimeter thermopile); the radiant calorimeter thermopile signal is independent of the location of the heat source in the chamber, and when referenced to a cold junction temperature of 20°C, it has a response of 0.0212 mV/g·cal/min; the ventilatory calorimeter thermopile has a response of 0.0159 mV/g cal/min when referenced to a cold junction temperature of 4°C; the automatic temperature control systems for the two thermopiles provide ±0.01°C control at their respective temperatures; the noise‐to‐signal ratio is less than 1% for the radiant calorimeter thermopile and less than 2% for the ventilatory calorimeter thermopile at thermal input levels comparable to those expected from the experimental animal.