The construction of an artificial circulation for the study of thrombolysis allowing variation in fluid pressure and flow rate is described. The lysis of 125I-fibrin was not influenced by variation in pressure between 10 and 30 cm H2O, but was significantly faster at a flow rate of 25 ml/min than at 40 ml/min or 10 ml/min. At equivalent concentrations as assessed on fibrin plates tissue activator induced greater thrombolysis than urokinase. Thrombolysis by both activators was diminished in the presence of plasma.