The enzymatic activation of amino acid was recognized and measured by the formation of hydroxamic acids in the presence of amino acids, adenosine triphosphate and hydroxylamine. Tyrosine-activating enzyme was partially purified from the cardiac muscle of dogs by the method of Schweet. The "cell-sap" fraction of the cardiac muscle of dogs formed hydroxamic acids of 18 kinds of amino acids. Tryptophan, tyrosine, cysteine, leucine, and proline were more strongly activated. In addition of the mixture of 14 kinds of amino acids, the mean μ moles of hydroxamic acids formed in/1.0g of wet weight/hour was 0.031 and 0.076 respectively for the cardiac muscle and the liver of normal rabbits. Mean activities of the enzyme of hypertrophied cardiac muscle of dogs caused by aortic valvular lesions and of thyrotoxic cardiac muscle of rabbits were higher than those of the control, while those of the starved, hypoxic and norandrostenolone administered cardiac muscle of rabbits were lower. There was no difference between the mean value of the acute cardiac dilatation and that of the control. It is suggested from these results that the protein synthesis from the amino acid-level is present in the cardiac muscle and that it is likely to be disturbed at the level of amino acid-activation under some pathologic conditions.