Young adult female rats, previously labeled with L-(14 C-guanido) arginine and L-(3-3 H) glutamic acid were fed ad libitum diets containing either 10- or 0-percent lactalbumin supplemented with arginine and glutamic acid, 0-percent protein or 10-percent lactalbumin plus amino acids restricted to 5 g diet/ day. Retention of radioactivity in liver and muscle was greater in the restricted groups but was restored to normal by feeding cold amino acid, indicating amino acid recycling. It is concluded that the apparent changes in rates of protein synthesis and degradation during protein and calorie restriction are due to changes in amino acid recycling rather than changes in metabolic rate, and that amino acid reutilization can be minimized by providing dietary amino acids.