Three experiments were performed to test the assumption that imbalanced dietary amino acid mixtures must lead to increased heat production. The first experiment was based on diets formulated to have a wide range of crude protein concentrations but a fixed concentration of lysine, formulated to be the first-limiting amino acid. In the second experiment, lysine concentration was varied over a wide range while CP content was kept constant. To prevent the masking of dietary effects by thermoregulatory demands, the third experiment was performed at 30°C with the diets similar to the diets used in the second experiment. The detailed relationships among amino acid balance, nitrogen metabolism and energy metabolism were investigated in a computer-controlled chamber calorimetry system. In experiment 1, there was a 75% increase in N intake as CP concentration increased. This led to a 150% increase in N excretion, with no significant change in HP. In experiment 2, there was a 3-fold difference in daily weight gain between the lowest and highest lysine diets HP per bird increased significantly with dietary lysine concentration. There was still an effect when HP was adjusted for body weight differences, but it failed to maintain statistical significance. In experiment 3, HP per bird increased significantly with dietary lysine content, whether or not adjusted for body-weight. The trend was greater than in the previous experiment (20°C). To investigate the effect of amino acid balance and protein quality on growth rate and carcass characteristics, growth trial experiment was performed on a larger scale. Four diets varying CP contents were used. The results showed that there was no significant effect of high protein diet on growth rate and carcass characteristics. A free-choice feeding experiment was also performed to investigate the sensitivity of the bird to its diet on the basis of amino acid balance, especially related to lysine concentration.