Procoagulant activity of the endothelium and of monocytes was induced by an intravenous administration of a low dose of endotoxin in the rabbit. Venous thrombosis induced by a 45-min stasis of the jugular vein was dramatically increased after the administration of endotoxin. Endotoxin enhanced venous thrombosis in a dose-dependent manner which was the highest after the administration of 1 µg/kg. This effect increased with time but was optimal when endotoxin was injected 4 h before stasis induction. Leukocytes appear to play a key role in this process since mechlorethamine-induced leukopenia totally inhibited thrombus formation. Hirudin and heparin, administered intravenously 5 min before stasis, were both potent inhibitors of endotoxin-induced venous thrombosis exhibiting ED50 values of 3.8 ± 1.0 and 2.3 ± 1.1 µg/kg, respectiv