The endothelium plays an active role in inhibiting and promoting the activation of coagulation. Loss of the endothelial lining results in a thrombogenic state. The inactivation of thrombin on rabbit aorta after a superficial and deep injury to the vessel wall was studied. The thrombin enzymatic activity on the subendothelium could be modulated and decreased activity was found on both types of injured vessel segments when exposed to antithrombin III (AT). An increased activity by exposing the superficially injured vessels with heparin was seen. This effect of heparin was not observed on the deeply injured vessels. Most probably, heparin elutes AT from the surface of the superficially injured vessels, but on the deeply injured surface no AT is present and, consequently, heparin has no effect in this respect. These results speak in favour of treatment with AT instead of heparin in cases of arterial injury.