SummaryDeterminations were made of the phosphoglyceric kinase activities exhibited by intima-media layers of various types of human blood vessels. A total of 445 vascular specimens was included in the study. The average activity of the thoracic descending aorta expressed in millimoles of 1,3-diphos-phoglycerate formed per gram fresh tissue per hour was .277; the corresponding values recorded for the ascending aorta, abdominal aorta, pulmonary artery, coronary artery, and inferior vena cava were, respectively, .276, .233, .298, .282, and .164. The mean enzyme activity exhibited by lipid-arteriosclerotic aortic tissue samples was 86.2% of that observed for normal tissue of the same arterial specimens.