The effect of physical training on thrombotic tendency was assessed in rats. Exercise was done on a flat treadmill for 30 min at a rate of 1,400 m/h (submaximal speed), 5 times a week for either 1.5 or 3 months. The thrombotic tendency was measured by the He-Ne laser-induced thrombus formation method in microvessels of mesentery, i.e. measurement of the number of laser irradiations necessary to induce stasis of blood flow by occlusive thrombus formation. An increase in the number of irradiations necessary to induce occlusive thrombus formation was observed in arterioles, but not in venules after physical training for 1.5 and 3 months.