Comparative studies of the chemistry and metabolism of human and bovine arterial and venous tissue had the following results (values for bovine tissue in parenthesis): 1. In arterial tissue the contents of collagen were 21% (37%) and of elastin 20% (31%) referred to the dry weight. In veins collagen amounted to 47% (30%) and elastin to 7% (18%). The mucopolysaccharide concentration was higher in arterial (1.2 and 1.5 resp.) than in venous tissue (0.4 and 0.2% resp.). Hyaluronate, chondroitin 6-sulphate (chondroitin 4-sulphate), dermatan sulphate and heparan sulphate were identified. In arterial tissue chondroitin sulphate was predominant with 59 % and 49 % resp. In human venous tissue the main component was dermatan sulphate (65%), in bovine veins it was hyaluronate (75%). No dermatan sulphate was found in bovine venous tissue. The number of cells was higher in arteries (4.5 × 107 nuclei/g w. wt.) than in veins (2.8 × 107). 2. On in vitro incubation bovine arterial tissue metabolized 73 %, venous tissue 32% of the added [U-I4C] glucose within 12 h. In this period 2.8 and 2.4 mg lactate resp./g wet tissue were formed by arteries and veins. The specific radioactivity of lactate synthesized in arterial tissue reached that of the added [U-I4C] glucose whereas lactate formed by veins had only 36 % of the specific activity of the [U-I4C] glucose. The specific radioactivity of the acid mucopolysaccharides was 5–6 times higher in venous tissue than in arterial tissue. After labelling with 35S-sulphate a 3-fold higher specific radioactivity of venous sulphated mucopolysaccharides was observed. 3. After incubation of bovine venous tissue in the presence of I4C-acetate the specific radioactivity of phospholipids was 45 % higher than that of arterial tissue in contrast to a 50 % lower labelling of cholesterol in venous tissue.