Hirudin is a potent inhibitor of thrombin, which was originally obtained from the medicinal leech (Hirudo medicinalis) Now it is being produced through the recombinant technology on a large scale. Recombinant hirudin has been assayed for the anticoagulant activity by the measurement of clotting time and the inhibition of thrombin actvity using a chromogenic substrate. The assay range of partial thromboplastin time and thrombin time is within $0.2{sim}1.0 {mu}g/mι.$ Thrombin time is more sensitive to the measurement of clot. Ex vivo study showed the level of hirudin in rat plasma was highest in 10 min and then it was eliminated slowly. The half-life of r-hirudin was 80~110 min depending on the assay methods. Intraveneous injection of russel viper venom was used for thrombus induction combined with vents cava ligation. Inhibition of venous thrombosis was observed with i.v. hirudin. It was dependent on the concentration of hirudin.