1 Insertion of a cotton thread into an arteriovenous shunt of an anaesthetized rat causes an increase in the weight of the thread due to deposition of thrombus. 2 The thrombus formed was of the venous (red) type, being sensitive to heparin, yet possessed important characteristics of an arterial thrombus, in that it was dependent on platelets and on rate of blood flow. 3 Thromboxane synthetase inhibitors had no effect on thrombus deposition. 4 Cyclo-oxygenase inhibitors did not significantly depress thrombus formation at doses which inhibited platelet function ex vivo. 5 Compounds which can modify the release or action of adenosine 5' -diphosphate partly inhibited thrombus formation. 6 A depression in clotting factor levels induced by sodium warfarin led to a highly significant reduction in thrombus formation at doses which caused a prolongation of prothrombin clotting time.