The myocardial metabolism of amino acids was observed by the use of the coronary sinus catheterization method, heart-lung preparation and heart muscle homogenate. In the normal dog heart in situ as well as in the heart-lung preparation of normal dog, neither extraction nor release of total amino acid nitrogen was observed and, on the contrary, respective extraction and release were observed as to individual amino acids. With administrating amino acid, individual amino acids were transformed actively each other in the heart muscle. There was, however, the difference of metabolic condition between the administration of glutamic acid and alanine. Based on the result obtained in the normal heart and in the heart undergoing various condition, glutamic acid and gluramine seemed to be the key substances of myocardial amino acid metabolism. The observation on amino acid activating enzyme was also described.