The effect of 3,5,3[image]-triiodothyronine (T3) on the intracellular accumulation of 20 amino acids by embryonic chick bone has been investigated. T3 had no effect on intracellular concentrations of the acidic and basic amino acids tested, but did influence the accumulation of certain neutral amino acids. At appropriate amino acid concentrations. T3 stimulated the concentrative accumulation of free intracellular tyrosine, phenylalanine, tryptophan, cycloleucine, histidine, leucine and methionine; inhibited that of glycine, valine and isoleucine; and did not influence that of other neutral amino acids tested. For each responsive amino acid, the effect of T3 on uptake disappeared if amino acid concentration was sufficiently increased. Previous studies have demonstrated the presence in embryonic chick bone of a minimum of 2 sites for the active transport of neutral amino acids, designated "A" and "L." The amino acids whose accumulation was influenced by T3 are those with relatively high specificity for transport at the L site, while those which were uninfluenced by T3 have high specificity for transport at the A site. Competitive blockade of transport at the L site prevented the effects of T3, whereas blockade of transport at the A site inhibited the effects only partially or not at all. The results are interpreted as indicating that T3 interacts specifically with the L transport site, causing changes in the affinities of affected amino acids for that site.