PURPOSE: To assess endothelial function after arterial thrombosis creation and after administration of a novel thrombolytic regimen in a new porcine model. MATERIALS AND METHODS: Untreated arteries that had undergone thrombosis for 90 minutes were compared to arteries treated with tissue plasminogen activator (tPA, 4 mg) and a combination of tPA and L-arginine (L-arg; 600 mmol/L). External iliac artery luminal diameter was measured with use of duplex ultrasonography. Endothelialdependent relaxation (EDR) and endothelial-independent relaxation (EIR) were measured with use of acetylcholine chloride (ACh) and sodium nitroprusside (NTP), respectively. Endothelial integrity was confirmed by scanning electron microscopy (SEM). Nitric oxide (NO) levels were determined with use of a chemiluminescent assay of its nitrite/nitrate metabolites (NOx). RESULTS: After thrombosis, EDR was decreased (69% ± 9.5; ACh = 15 μg/min; n = 6). EDR remained unchanged after thrombolysis with tPA despite complete dissolution of thrombus (67% ± 5.7; ACh = 15 μg/min; n = 5). Thrombolysis with use of tPA coupled with L-arg resulted in an increase in EDR (95% ± 4.9; ACh = 15 μg/min; n = 5; P = .007). EIR was preserved in all groups, with uniform response to NTP. SEM analysis revealed intact endothelium in all groups. Local NOx levels were diminished after 90 minutes of thrombosis (49.3 μmol/L vs 40.8 μmol/L; P = .0002), but increased to 55.7 μmol/L after thrombolysis with tPA and L-arg (P = NS). CONCLUSIONS: Thrombus induces arterial dysfunction acutely without altering endothelial integrity. This dysfunction is ameliorated through regional administration of L-arg in combination with standard thrombolytic therapy, which increases local NO levels. This model allows the in-vivo study of thrombosis and alternative thrombolytic regimens. Regional enhancement of NO levels may prove to be an attractive adjunct in thrombolytic therapy.