In order to investigate the effect of dietary protein and fat levels on the growth and the utilization of nitrogen and energy and body composition in rats, SpragueulcornerDawley 48 male rats of 8 weeks old weighing approxijIlately 215-220g were subjected to feeding trials for 8 weeks and then subsequently to metabolic trials for 2 weeks. Four dietary protein levels (4, 8, 16, 32%) and each protein level contained two fat levels(3.9, 11.7%=1O,3J% of 3600kcal ME/kg) by addition of an appropriate amount of carboulcornerhydrate and the following results were obtained. The body weight gain and food efficiency ratio of the rats to which a diet of 16% protein and 3.9% fat was fed were significantly higher than in either case of 8% proulcornertein diet or of 32% protein diet. The digestibility of protein in the experimental diets was 73.3 -93.4%. The digestiulcornerbility of energy ( energy absorption) in the experimental diets was 83.2 -91.5%. The utilization of protein and the metabolic energy efficiency in the experimental diets was highest at the diet of 8% protein and 3.9% fat. The analysis of the body composition after feeding trials for 8 weeks has shown that the content of body water and protein were not affected by protein and fat levels in diet. The content of body fat in the rats to which 3.9% fat diet was fed was highulcornerer than that in those to which 11.7% fat diet was fed. From the above experimental results it may be suggested that the best formula of diet for the 8 weeks old rats may be composed of the 8% protein and 3.9% fat.