1. Leg blood flow, uptake of oxygen and glucose and release of lactate by the leg and changes in intramuscular concentrations of metabolites were studied at rest and during exercise of increasing work loads in thirteen patients with occlusive disease of the iliac or superficial femoral arteries. 2. Leg blood flow (dye-dilution technique) and oxygen uptake during exercise were low and levelled with increasing work load. Considerable increases were noted in muscle lactate concentration and in the net release of lactate from the exercising leg. Muscle content (needle-biopsy technique) of ATP and creatine phosphate decreased during exercise, with an almost complete depletion of creatine phosphate in three patients. The decrease in muscle glycogen during work did not differ significantly from that of control subjects. 3. Repeated exercise after reconstructive surgery showed a considerable improvement in physical working capacity. Leg blood flow and oxygen uptake during exercise were significantly higher than before surgery and increased linearly in relation to work intensity. The decrease in creatine phosphate and lactate concentration of the thigh muscle during exercise was less pronounced and the release of lactate was lower than before vascular reconstruction. 4. It is suggested that the onset of the severe muscle symptoms during exercise in patients with occlusive arterial disease of the leg may be related to a low concentration of ATP and creatine phosphate in the affected muscles.