Exercise and post-exercise metabolism was studied in the legs of 6 healthy individuals without arterial occlusive disease and in 34 patients with intermittent claudication. Venous blood from the popliteal vein was taken intermittently by regional catheterization during and after an exhaustive constant load exercise on a treadmill. Change of lactate and pyruvate was studied in arterial and popliteal venous blood. The mean arterial concentration of pyruvate during exercise was increased less in patients and controls as compared to the lactate concentration in both groups. There was a continuous rise of the arterial pyruvate concentration after exercise in patients in contrast to the controls. During exercise, the difference between the arterio-popliteal venous blood samples in lactate concentrations was significantly higher in patients than in the controls. In general, patients had a significantly higher lactatepyruvate ratio in the popliteal venous blood than in the arterial blood. It is concluded that the onset and degree of anaerobic work is best studied by the regional catheterization technique. This new technique permits to study the spontaneous and reactive metabolic changes in the legs during and after treadmill exercise in patients with intermittent claudication.