The thrombogenicity of the vessel wall after endothelial denudation is partly explained by an impaired inhibition of thrombin on the subendothelium. We have previously reported that thrombin coagulant activity can be detected on the vessel wall after balloon injury in vivo. The glycosaminoglycans of the subendothelium differ from those of the endothelium and have a lower catalyzing effect on antithrombin III, but inhibition of thrombin can still be augmented by addition of antithrombin III to the injured vessel surface. In this study the effect of antithrombin III and heparin on thrombin coagulant activity on the vessel wall was studied after in vivo balloon injury of the rabbit aorta using biochemical and immunohistochemical methods and thrombin was analysed after excision of the vessel. Continuous treatment with heparin, lasting until sacrifice of the animal, or treatment with antithrombin III resulted in significant reduction of thrombin coagulant activity on the injured aorta. Heparin given only in conjunction with the injury did not prevent thrombin coagulant activity or deposition of fibrin on the surface. The capacity of the injured vessel wall to inhibit thrombin in vitro was improved on aortic segments obtained from animals receiving antithrombin III but not from those given heparin. It is concluded that treatment with antithrombin III interferes with thrombin appearance on the vessel wall after injury and thereby reduces the risk for thrombosis.