Human calorimeter studies have been made in the past under static environmental conditions since stabilization time or heat content of thermal barrier materials limit measurement when the environment is changed. Direct calorimetry combined with indirect calorimetry offers the marked advantages of measuring the change in heat content of the body during transient states. Thus the most significant value of direct calorimetry is in dynamic caloric studies where transient energy exchange is measured. The thermal gradient calorimeter as devised by Day and Hardy(l) and Benzinger and Kitzinger(2) permits these studies when the unsteady state is produced by conditioning of the subject at different temperatures prior to insertion into the calorimeter or by increasing heat production by exercise. The human calorimeter described is designed to indicate variations in body heat exchange and to allow change and stabilization of environment within a few minutes. It is similar in concept to the calorimeter described by Visser and Hodgson(3). Overall design and performance specifications.† The interior floor space is .91 meter by .76 meter and the height is 1.83 meters allowing the subject to sit or stand in the calorimeter. Inlet temperature can be regulated at temperatures between 5°C and 50°C. Humidity may be reduced from ambient conditions by a precool coil. The temperature can be changed at a maximum rate of 2.5°C per minute.