The role of hemodynamics on platelet thrombus formation was studied in venules and arterioles of the rat mesentery. Thrombus formation was induced by the fluorescent dye/light method for examination of the following factors: 1) the effect of wall shear rate on thrombus initiation, 2) the effect of wall shear rate on the growth of thrombi, and 3) the relation between platelet thrombus initiation and intraluminal velocity profile. The range of wall shear rate was up to approximately 1,000 1/sec in venules and from 640 to 2,900 1/sec in arterioles. Platelet thrombus initiation occurred more rapidly at higher wall shear rate in venules and at lower wall shear rate in arterioles. Thrombus initiation time was shortest around a wall shear rate of 900 1/sec in venules and around 700 1/sec in arterioles. Thrombus growth rate in venules was greatest at a wall shear rate of 1,500-2,000 1/sec. Thrombus initiation and its relation to blood flow was also examined in branched and curved microvessels. In these vessels platelet thrombi were also first initiated at the sites of higher wall shear rate in venules and of lower wall shear rate in arterioles.