It is well known that undue pressure and prolonged use of tourniquets on an extremity can cause peripheral nerve palsy and vascular damage either from direct pressure or from acid metabolites accumulating in the tissues distal to the tourniquet applied. After release of a tourniquet that has been on for 2 hours, the acidotic changes in the limb take approximately half an hour to recover, and it has been suggested that two hours' ischemia should never be exceeded. The authors have conducted a study to clarify the effect of these acid metabolites on the systemic circulation upon release of tourniquet after 105 minutes (mean) use. In 12 orthopedic patients, 20~32 years of age, who underwent operations below the knee, anesthesia was induced by the intravenous thiopental-succinylcholine-endotracheal intubation sequence and maintained with nitrous oxide-oxygen-halothane in a semiclosed circle absorption system. Respiration was controlled throughout the procedure. Measurements of PO₂, PCO₂ and pH in the femoral vein and radial artery before inflation of a tourniquet and 15 minutes, 10 minutes, 20 minutes, and again 30 minutes were performed following the release of tourniquet. The results are as follows: 1) Femoral vein pH significantly decreased with concommitant increase in PCO₂ reflecting severe metabolic acidcsis during the tourniquet time. 2) Femoral vein PO₂ was significantly increased, suggesting the presence of A-V shunt in the extremity. 3) These changes continued to exist for approximately half an hour after release of tourniquet. 4) Analyses of radial arterial blood for PO₂, PCO₂ and pH revealed no significant changes throughout the study. 5) It is concluded that acid metabolites produced by tourniquet application do not seriously affect when tourniquet is released as long as the buffering capacity and blood volume are kept normal and the patient is kept alkalemic by controlled ventilation.