The interaction of platelets and the coagulation system has been investigated in vivo on a cotton thread implanted in an arteriovenous shunt in the rat. Depletion of the circulating platelet count with anti-platelet antibody reduces both thrombus deposition and 125I-labelled fibrinogen incorporation demonstrating the platelet behaviour with either anagrelide or prostacyclin decreases thrombus deposition by a greater extent than that expected from the platelet content of the thrombus. Treatment with warfarin also dramatically decreases both formation and 51Cr-labelled platelet deposition demonstrating the important role of the coagulation system in platelet aggregation in this model. Treatment with Arvin or with heparin also reduced the thrombus weight. It is concluded that this is a model of bidirectional platelet interaction with the coagulation system and may have similarities with thrombosis which occurs in extracorporeal shunts in man.