The metabolic fates of the carbon skeletons of leucine, lysine, and threonine were studied in growing rats on the diets containing graded levels of protein calorie percentages (10, 20, 30, and 40 PC%) by use of either gluten or zein at 4100 kcal of metabolizable energy per kg of diets. In growth experiment for 21 days, body weight gain, food intake, and body fat increased at higher PC% in the gluten diets, but rats given zein did not maintain their initial weight even at 40 PC%. The concentration of plasma free lysine remained low with the zein diets, but plasma threonine increased at 10 and 20 PC% in the gluten and zein diets, respectively. Plasma leucine increased as the protein level increased either dietary protein. More than 70% of 14C was incorporated into body protein 12 h after an intraperitoneal injection of labeled lysine in all groups, but little 14CO2 was expired in rats on the gluten and zein diets. About 79% of 14C-threonine was incorporated into body protein in rats given the gluten and zein diets at 10 PC%, but the values were gradually decreased with increasing the dietary protein levels. Some 40-50% of 14C-leucine was incorporated into the body protein in rats given the gluten diets, and the values for the zein diets were extensively decreased in the higher PC% groups where the expired 14CO2 was inversely increased to a great extent. These results showed that, when a specific amino acid was limiting or deficient in the diet, the major portion of the labeled amino acid was utilized for body protein synthesis and little was oxidized to carbon dioxide, whereas the oxidative degradation of essential amino acid other than limiting one was increased and the efficiency of the amino acid utilization was relatively decreased.