The plasma individual amino-acid concentrations in the rat during late foetal development and from the birth to the end of the weaning have been studied. Both the very low foetal amino-acid degrading capabilities in addition to the high amino-acid concentration ability of the placenta help to sustain considerably high plasma foetal amino-acid levels that favour considerably protein synthesis. There is a very important decrease in most amino-acid concentrations with birth, due to the cessation of placental transfer, relative immaturity of intake and assimilation processes and to the highly anabolic environment found in the pup. The changes in postnatal development plasma amino-acid patterns reflect directly the availability of amino acids in the diet, the maturation of the amino-acid metabolism pathways and transport systems and their use for protein synthesis (and other nitrogenous compounds). The main situation that affects individual amino-acid levels is the shift from high biological value milk protein to low biological value plant proteins in the rat chow pellets. The postnatal development is characterized by a high combined total amino-acids homeostasis with ample changes in individual amino-acid concentrations, that is maintained in spite of the deep changes in diet, size, metabolism, hormone environment, maturation, etc. observed in the rat from birth to weaning and adulthood.