Respiration experiments were conducted by the open-circuit, Haldane respiratory quotient procedure, with 48 mature albino rats as subjects, to investigate the energy expense of utilization (heat increment) of complete diets as affected by their contents of fat. Heat increments were measured as the difference in heat production from maintenance and supermaintenance diets containing 2, 5, 10 and 30% of fat, respectively, fed in such a way as to supply equal quantities of gross energy, of protein, and of vitamins. In harmony with results of a growth experiment in which the same diets were fed, the digestibility and the retention of food nitrogen were highest when the diet containing 30% of fat was used. The metabolizable energy of the diets was unaffected by their fat contents. The heat production at both planes of nutrition, and also the heat increments, diminished in the order of the increasing fat contents of the diets. The heat increments of the dietary supplements containing 2, 5, 10 and 30% of fat, respectively, were equivalent to 36, 31, 29 and 20%, respectively, of their gross energy. The decreasing energy expense of utilization of the isocaloric diets, in the order of their increasing fat contents, was due to decreasing heat from the catabolism of carbohydrate and from fat synthesis.