Recent in vitro studies have demonstrated that thrombin inhibits fibrinolysis through thrombin-activatable fibrinolysis inhibitor (TAFI, plasma procarboxypeptidase B). We have recently shown that endogenous fibrinolysis in vivo is enhanced by activated protein C (APC) and the selective thrombin inhibitor, argatroban. The aim of the present study was to examine the role of TAFI in these fibrinolytic mechanisms in vivo using purified porcine pancreatic carboxypeptidase B (PPCPB) and a specific TAFIa inhibitor, potato tuber carboxypeptidase B inhibitor (PTCI) in a newly established arterial thrombolysis model. Non-occlusive, mural, platelet-rich thrombi were formed by helium-neon laser irradiation in rat mesenteric arterioles and thrombus size was measured by computerised image analysis. We confirmed that endogenous thrombolysis was enhanced by argatroban (2.0 mg/4 ml/kg/h) or APC (1.62 mg/ 2.31 ml/kg). PTCI (5.0 mg/2 ml/kg) also accelerated endogenous thrombolysis. PPCPB (3.5 mg/2 ml/kg) inhibited thrombolysis in the absence and presence of argatroban or APC. PTCI tended to further promote APC-induced thrombolysis but the differences did not reach statistical significance. The present findings were in keeping with the results of earlier studies and demonstrated that arterial, platelet-rich thrombi in vivo are degraded by naturally generated plasminogen activators. TAFI may play a significant role in the control of these mechanisms.