The arterial inflow into the calf, venous emptying rate and venous capacity were studied by means of venous occlusion plethysmography on the non-operated leg in two groups of patients undergoing total hip replacement. One group of eight patients received epidural blockade intraoperatively, which was prolonged postoperatively for pain relief, while the other group of eight patients was given general anaesthesia with intermittent positive-pressure ventilation intraoperatively and parenteral analgesics for pain relief in the postoperative period. The calf blood flow and venous capacity were lower in patients receiving general anaesthesia when measured at the end of surgery and significantly lower in this group when measured 3 h postoperatively. The sustained reduction of flow in the deep veins of the lower limb might be a significant contributory factor in the initiation of deep venous thrombosis. In three of the eight patients in the general anaesthesia group, the venous emptying rate and venous capacity had decreased 3 h postoperatively to a very low level, indicating thrombus formation in immediate association with the surgery.