Common femoral arterial and bypass blood flows, cardiac output, brachial arterial and central pressures were measured in nine patients undergoing a femoro-popliteal vein bypass graft for intermittent claudication. The measurements were performed during neuroleptanesthesia before the operation, after the operation before blood substitution and then after transfusions of 450,900 and 1350 ml of blood. The leg blood flows were measured before and during local vasodilatation induced by intra-arterial injection of bamethane (Vasculat). The bypass flow was only 55 ml/min before blood substitution but was increased to 215 ml/min after the blood transfusions. The corresponding values for the common femoral artery were 100 ml/min and 430 ml/min, respectively. The maximal flow in the bypass vein during vasodilatation with bamethane (Vasculat) was 145 ml/min before blood substitution and 270 ml/min after the transfusions. This implies that the flow response to vasodilatation was more pronounced before than after blood substitution. The leg blood flow increased proportionally more than cardiac output on blood substitution. It is concluded that leg blood flow is very sensitive to hypovolemia and can be increased considerably by adequate blood substitution. The flow response to local vasodilatation is proportionally higher during hypovolemia.