The intravascular platelet aggregation and the subsequent thrombus formation were found to be induced in the microvasculature by the irradiation of filtered light in combination with the intravascular administration of fluorescein sodium. Effects of several factors such as light intensity, dye concentration and vessel diameter were examined in venules of rat mesenteries. It was confirmed that the platelet thrombus was induced with excellent reproducibility depending on the combined condition of the light intensity and the dye concentration. In venules within the range of 20 to 80μm in diameter, almost the same values of the initiation time of thrombus formation were obtaind. On the other hand, the time to totally occlude the vessel lumen by the thrombus prolonged significantly with the increase of vessel diameter, which phenomenon seemed to be induced by almost the same growth rate of thrombus in the radial direction in all venules with different diameters. Inhibitory effects of prostaglandins (PGD₂, PGE₁) for the thrombus formation were also examined by the proposed model. The platelet aggregation in venules was significantly inhibited by higher concentration of these doses. This result well corresponded to that of in vitro experiments reported by other researchers. The present model seems useful for various purposes in studying the kinetics of intravascular thrombus formation as well as evaluating the effects of anti-platelet aggregating drugs.