The role of the local synthesis of thrombin in platelet recruitment and thrombus stabilization in heparinized blood was examined in vitro. Mural thrombosis was visualized and measured in a thin, rectangular, collagen-coated capillary under controlled rheological conditions by using fluorescence digital videomicroscopy and fluorescence microphotometry. Thrombin activity was inhibited in heparinized blood by the synthetic competitive inhibitor, D-phenylalanyl-L-prolyl-L-arginyl chloromethylketone (FPRCH2Cl), resulting in a marked reduction in the rate of platelet accumulation on collagen surfaces, indicating a role for thrombin in platelet recruitment. Similar although lesser effects were observed by reducing thrombin synthesis with antibodies to factors II and X. To decouple the role of thrombin in platelet recruitment by direct stimulation of platelet activity from its role in thrombus stabilization via fibrin formation, thrombosis was measured in heparinized blood treated with the tetrapeptide glycyl-prolyl-arginyl-proline, which inhibits fibrin monomer assembly into fibrin. The ultimate level but not the initial rate of platelet accumulation was reduced markedly, indicating a role for fibrin in thrombus stabilization against hemodynamic forces. Scanning electron micrographs demonstrated fibrin stands in the heparinized control samples but not in the heparinized samples with glycyl-prolyl-arginyl-proline. These results demonstrate a role for the local action of thrombin synthesized on the surfaces of thrombi even under conditions when the thrombin exerts no bulk effect, such as under heparin anticoagulation. Furthermore, this role appears to be a result of both platelet recruitment and thrombus stabilization. [on SciFinder (R)]