(+)-2S-2-[4-[[(3S)-1-Acetimidoyl-3- pyrrolidinyl]oxy]phenyl]-3-[7-amidino-2-naphthyl]propanoic acid hydrochloride pentahydrate (DX-9065a) is an antithrombin III (AT III)-independent and selective inhibitor of activated blood coagulation factor X (FXa). The aim of the present study was to compare the antithrombotic and hemorrhagic effects of DX-9065a with a direct thrombin inhibitor and AT III-dependent anticoagulants in rat models of thrombosis and bleeding. Rats were administered intravenously DX-9065a (0.1-1 mg/kg/h), argatroban (0.1-1 mg/k/h), low molecular weight heparin (25-100 anti-XaU/kg/h), unfractionated heparin (25-100 anti-XaU/kg/h) or Orgaran (30-300 anti-XaU/kg/h) for 1 h. DX-9065a dose-dependently inhibited both thrombus formation and elevation in plasma thrombin-AT III complex (TAT) level in a copper wire-inserted arteriovenous (AV) shunt model in rats. The dose required for 50% inhibition of thrombus formation was 0.27 mg/kg/h. DX-9065a did not prolong transection bleeding time up to 7.78 mg/kg/h. Argatroban and AT III-dependent anticoagulants also inhibited both thrombus formation and TAT elevation, but prolonged bleeding time at a slightly higher dose than the effective dose. These results suggest that direct and selective inhibition of factor Xa by DX-9065a is preferable for the treatment of thrombosis in the aspect of lack of compromising primary hemostasis.