The role of the collagen receptor glycoprotein VI (GPVI) in arteriolar thrombus formation was studied in FcRγ-null mice (FcRγ–/–) lacking platelet surface GPVI. Thrombi were induced with severe or mild FeCl3 injury. Collagen exposure was significantly delayed and diminished in mild compared with severe FeCl3 injury. Times to initial thrombus formation and vessel occlusion were delayed in FcRγ–/– compared with wild-type mice after severe injury. Platelet accumulation in wild-type mice was decreased after mild compared with severe injury. However, there was little difference between platelet accumulation after severe or mild injury in FcRγ–/–. These data indicate a significant role for GPVI in FeCl3-induced thrombus formation. Pretreatment of wild-type mice with lepirudin further impaired mild FeCl3-induced thrombus formation, demonstrating a role for thrombin. Laser-induced thrombus formation in wild-type and FcRγ–/– was comparable. Collagen exposure to circulating blood was undetectable after laser injury. Normalized for thrombus size, thrombus-associated tissue factor was 5-fold higher in laser-induced thrombi than in severe FeCl3-induced thrombi. Thus, platelet activation by thrombin appears to be more important after laser injury than platelet activation by GPVI-collagen. It may thus be important when considering targets for antithrombotic therapy to use multiple animal models with diverse pathways to thrombus formation.