The effect of the application of a tourniquet to a limb and the release of the accumulated metabolites have been investigated with reference to the acid-base level in the blood from the limb and in the right atrium. Investigations have been carried out experimentally in rhesus monkeys and observations have been made on patients undergoing reconstructive operations on the knee. The acidotic blood from the ischaemic limb produces little systemic effect. The limb recovers in approximately 40 minutes after a tourniquet has been in place for four hours. Three hours is recommended as a reasonable upper limit for the safe application of a pneumatic tourniquet.