Antiplatelet drug aspirin and anticoagulant low molecular weight heparin (LMWH) were compared as arterial and venous antithrombotic preparations in the rat experimental model of the laser induced thrombus formation. A method to induce microthrombi in small mesenteric vessel (15-25 microns) has been developed to investigate antithrombotic drugs and to study platelet reactions. Mesenteric injuries are induced in the vascular system of Wistar rats with an argon laser. The laser beam induced formation of the vessel wall injury with damage of endothelial cells. Thrombus was formed within seconds after laser injury and grew rapidly. The aggregate can be swept away by the flow and a new thrombus was formed again. This embolization began within the minute following the laser flash. Thrombus formation and embolization were repetitive phenoma. Aspirin (100 mg/kg) and LMWH (1 mg/kg) are approximately the same as to decrease the number of emboli detached from the thrombus and the duration of embolization; both in venules and in arterioles. This results suggest reflexion about the role of platelets in venous thrombosis induced by laser beam.