Anin vitroperfusion system was used to investigate platelet interaction with subendothelium from rabbit aorta exposed to blood under controlled flow conditions. A morphological technique was used to measure platelet adhesion and thrombus formation. Classical mass transport theory modified to account for the dependence of platelet diffusivity on wall shear rate was used to analyze the results. Platelet adhesion increased with wall shear rate(10–650 sec−1),red cell concentration (10–70%), and platelet concentration(50–300 nl−1)and decreased with axial distance (0–20 mm) from the leading edge. Under these flow conditions platelet adhesion rate was determined predominantly by diffusional transport of platelets to the vessel surface. As shear rate increased to10,000 sec−1,a transition from diffusion to a more kinetic rate limiting adhesion was observed. Few thrombi were observed at low values of platelet concentration (<150/ml), red cell concentration (<25%), or wall shear rate(<200 sec−1).The formation of thrombi increased continuously with increasing wall shear rate to10,000 sec−1even in the region where values of platelet adhesion became relatively independent of shear rate. Thrombus formation was enhanced by an increase in red cell or platelet concentration and was significantly greater on the upstream portions of vessel segments.