Factors determining thrombus formation on a foreign surface were studied with the use of plastic flow chambers introduced into extracorporeal shunts. Silicone rubber shunts, joining the carotid artery and jugular vein, were implanted in dogs and remained patent for several weeks. The flow chamber geometry consisted of a 4.8 mm diameter straight tube having a 3.2 x 3.2 mm circumferential cavity in the wall. Chambers were introduced sequentially into the shunts for exposure times of 10 to 30 minutes and regulated blood flow rates of 100 to 400 ml/min. The dry weight of thrombus accumulated in the chamber (5 to 50 mg) was found to increase with exposure time up to 20 minutes and to decrease with increasing flow rate. Various components of the process of thrombus formation were altered by the administration of acetylsalicylic acid, heparin and lysozyme, used alone and in pairs. Heparin was found to be the most effective antithrombotic agent, dry weights of accumulated thrombus being of the order of 50 percent lower when compared to control values. The efficacy of heparin was found to be unaffected by the presence of aspirin and lysozyme, which themselves were not effective antithrombotic agents under the conditions of these experiments. The technique described here may provide a useful animal model for studying the influence of blood flow and different biomaterials on thrombus formation.