The response of adult rats fed diets containing amino acid mixtures with varying combinations of lysine and threonine was studied by estimating the change in body weights, organ weights, total nitrogen content of organs and the concentration of free plasma lysine and threonine. Analysis of variance indicates a significant effect of threonine, lysine and a significant interaction due to feeding the two amino acids on the body weights, organ weights and total nitrogen content of various organs. Significant effect on the level of the plasma free amino acid was also shown as a result of feeding the particular amino acid. Results show that in deficient animals the rates of building body protein are low but are higher in the lysine deficient animals than in the threonine deficient and protein deficient animals. The varying rates of exchanging protein between the various organs in response to changing the levels of amino acids in the diet would mean that total change in body protein or total nitrogen balance may not be satisfactory way to determine the protein or amino acid requirements for maintenance. It seems to be necessary to focus down on protein turnover rates in specific tissues.