A lysine-deficient amino acid mixture was developed by decreasing the level of lysine in a well-balanced amino acid mixture while measuring the response in daily gain and protein retention in young growing rats. The diet that contained the highest level of lysine that did not support maximal growth or protein retention was used as the lysine-deficient amino acid mixture. This mixture was incorporated into a series of diets to provide 0.52, 0.59 or 0.67% of lysine in combination with 0, 25 or 50% relative excess of all other amino acids. Food intake, weight gain and changes in carcass composition were measured in the 21-day feeding study. When voluntary food intake was used as a covariate in the statistical analysis, increased dietary lysine improved the efficiency of food use for body weight, dry matter, crude protein and ash gains. When corrected for food intake, lysine level did not affect lipid retention. There was no effect of excess amino acids on food intake, daily weight gain or body component gains whether food intake was used as a covariate or not. These results suggest that growing rats can respond to improved amino acid nutriture with changes in the efficiency of food use rather than improvements in growth.