A 70-day metabolism and body analysis experiment was conducted to determine the effects of differences in the fat content of isocaloric diets on the utilization of food energy and protein. The subjects were four groups of 10, growing male albino rats, each of these four groups containing one rat from each of the same ten litters. A comparison was made of four diets containing 2, 5, 10 and 30% of fat, respectively, these diets being so compounded and fed as to supply to each rat of a litter-four the same quantities of gross energy, protein, and vitamins. Determinations were made of gains in live weight, nitrogen, fat and energy, with a single value of the heat production for the 70 days as the energy of the food minus the energy of the excreta and of the body gain. The gains in live weight, the digestibility of nitrogen, and the retention of nitrogen and energy were in the order of the increasing fat content of the diets; the superiority of the 5% over the 2% fat diet with respect to the utilization of both protein and energy being much greater than the superiority of the 30% as compared with the 5% diet.