Plasma beta-thromboglobulin (beta-TG), a marker of in-vivo platelet release reaction, was measured in 27 meniscectomy patients, 10 patients who underwent total knee replacements (TKR) (both procedures performed under tourniquet ischaemia), and 10 herniorrhaphy patients. In 22 meniscectomy and 7 TKR patients plasma-thromboxane-B2 (TxB2), a stable end-product of thromboxane-synthetase activity, was also determined. The mean plasma beta-TG and TxB2 had risen significantly within 5 min of release of the tourniquet, indicating increased in-vivo platelet release reaction and prostaglandin synthesis in these patients. In the herniorrhaphy patients beta-TG and TxB2 did not alter significantly postoperatively. In 6 pigs (weighing 20-30 kg) who underwent meniscectomy under tourniquet ischaemia plasma TxB2 rose significantly 15-30 min after release of the tourniquet, and prostacyclin release was significantly lower from veins exposed to ischaemia than from control veins. Large de-endothelialised areas were seen on scanning electron-micrographic sections of affected veins, and these areas were covered with a monolayer of platelets and platelet clumps. These changes associated with tourniquet ischaemia may explain the high incidence of venous thrombosis seen in patients undergoing meniscectomy and TKR.