Voluntary intake of proteins of various qualities in relation to dietary protein utilization was investigated in growing and adult rats. The rats were given two diets, one containing high protein and the other no protein, and were allowed to self-select protein and energy intakes freely from both diets. The results showed that total food intake (protein diet plus protein-free diet) and body weight gain were similar among five proteins tested. However, the amount of protein consumed by the growing rats per 100 g of total food intake (i.e., dietary protein level) was different depending upon the protein qualities, that is, wheat gluten (WG) 44 g, casein (CA) 30 g, soy protein (SP) 21 g, lactalbumin (LA) 19 g, and amino acid mixture simulating egg protein (AA) 11 g. Net protein utilization (NPU), estimated as the proportion of protein intake that is retained in the body, was as follows : WG, 20 %; CA, 33 %; SP, 44 %; LA, 50 %; and AA, 74 %. From the above figures, net dietary protein value, which is a measure of utilizable protein in the diet, was calculated by multiplying the dietary protein level by NPU. In contrast to the difference in protein intake, net dietary protein value was quite constant in spite of large differences in the dietary protein quality, being 8 to 10 %. A similar relationship between protein intake and protein utilization was obtained also with adult rats, except that the net dietary protein value was smaller in adult rats than in growing rats. These results may suggest that the animals can regulate the intake of dietary protein to keep the amount of protein available for the body constant.