In recent years the measurement of the heat output of animals has taken a prominent place in investigations on the physiology of nutrition, and especially so as regards farm animals. As it is now accepted that the conservation of energy applies to the chemical changes in the living body, the total metabolism can be measured equally well by the indirect or by the direct method. The indirect method using the Douglas bag has been shown by Benedict and others to be simple and satisfactory in the case of human beings, but it is not so easily applied to farm animals, and for these the direct method by the use of a calorimeter has advantages.