The incorporation of 14C-labelled amino acids into proteins of rat liver, kidney, brain, muscle and spleen was studied at the age of 10, 15, 25, 40 and 120 days. It was found that protein synthesis is very high in kidney, muscle and brain in young animals and decreases during the postnatal period. The rate of amino acid incorporation into proteins of liver and spleen decreases slowly up to 25 days andrises again at the age of 40 days. The administration of thyroxine (5 and 50 µg/100 g of body weight daily during 7 days) stimulates slightly the protein synthesis in liver, kidney and muscle, but has no effect in brain and spleen. The stimulation of protein synthesis by thyroxine during the postnatal period is related to the age of the animals. Differences in the increase of protein synthesis after thyroxine treatment were found in young and adult animals, the young rats being more responsive to thyroxine; the hormone increases the protein synthesis in suckling rats also in brain and spleen.