The oxygen tension in blood from varicose veins was compared with that in blood from normal superficial and deep veins of the legs. The investigations were made on 15 women and 7 men, age 26-70 years, with varicose veins in one or both legs. Blood was sampled from 2 to 4 varicosities along the saphenous vein in each affected leg and from corresponding sites in contralateral nonvaricose saphenous veins. Samples were also drawn from the popliteal vein in all legs. In a control group of 4 women and 5 men, age 34-63 years, blood was sampled from the saphenous and popliteal veins. No difference in oxygen tension of popliteal vein blood was found between the patients and the controls. Blood from varicose veins had significantly lower oxygen tension than normal venous blood. It seems unlikely, therefore, that arteriovenous shunting of blood is a general factor in the development of primary varicose veins of the legs.