A comparison was made of activities of 15 different enzymes in normal tissue samples of the inferior vena cava and aorta derived from the same subjects. Statistically significant differences between venous and arterial specimens were observed for 10 of these enzymes. Comparative studies on the intermediary metabolism of the vascular walls may disclose some of the factors responsible for the low susceptibility of human veins and the high susceptibility of human arteries to sclerotic changes.