To compare the fibrinolytic activity between the renal artery and vein and the systemic circulation, we measured tissue-type plasminogen activator (t-PA), urokinase and total fibrinolytic activity in the blood samples from both the left and right renal artery and vein and the anterior cubital vein of 7 kidney donors. Englobulin fibrinolytic activity was significantly higher in the renal vein [106.6 ± 5.6 blood activator units (BAU)] than in the renal artery (90.6 ± 4.1 BAU; p < 0.001) and cubital vein (94.3 ± 6.3 BAU, p < 0.005), but there was no difference between renal artery and cubital vein. t-PA Ag was 2.9 ± 1.1 ng/ml in the renal vein, 2.6 ± 1.0 ng/ml in the renal artery and 2.6 ± 1.1 ng/ml in the cubital vein. There was no difference between renal artery, renal vein and cubital vein. Urokinase was significantly higher in the renal vein (3.3 ± 0.4 ng/ml) than in the renal artery (2.4 ± 0.4 ng/ml; p < 0.001) and cubital vein (2.6 ± 0.4; p < 0.005), but there was no difference between the renal artery and cubital vein. In all cases, there was no difference in the fibrin(ogen) degradation product concentration between the renal artery and vein and the cubital vein. These findings suggest that the kidney may be an essential organ for providing urokinase to the systemic cir