Post-absorptive changes of the plasma amino acids reflect the adequacy of the balance of the dietary amino acids. Previous studies conducted in our laboratory utilizing man or dogs as test subjects have been successful in developing an in vivo plasma amino acid (PAA) ratio procedure to assess the ideal intake of dietary amino acids. Subsequent studies adapted this PAA ratio procedure to the rat in order to make it possible to determine the amino acid requirements for this species. By using the rat, it will be possible to determine the validity of these requirement data by carrying out long-term growth and balance studies. During the initial 9 months of this program, the amino acid requirements for the one and three-month old rat have been determined by the PAA ratio procedure. The data show a lower amino acid need for lysine and the sulfur amino acids for the one-month old rat and for lysine, threonine, valine, and isoleucine for the three-month old rat than the requirement data published in the literature. Similar data will be determined for the six and twelve-month old rat. Then long-term growth and development studies will be carried out to determine whether these ideal dietary amino acid patterns as assessed by the PAA ratio technique are more exact in predicting amino acid needs than the requirements as published in the literature & obtained by the nitrogen balance procedure.