Tissue-type plasminogen activator (t-PA) is a key enzyme in the regulation of fibrinolytic activity in plasma. The most important source of t-PA in vivo arethe endothelial cells of the vessel wall. Most authors believe that t-PA isreleased from veins, but it is not yet known whether it is released from arteries, too. The aim of our study was to check, by means of arterial occlusion, whether t-PA is released from the arterial wall and in what extent.Nine healthy young men were studied. We used a sphygmomanometer cuff toperform a short time occlusion of the cubital artery and compared the concentrations of t-PA antigen (t-PA Ag) and plasminogen activator inhibitor 1antigen (PAI-1 Ag) in the samples taken from the occluded and the contralateral brachial artery during the 5th to the 7th minute of occlusion. In a separated experiment the volume blood flow in the brachial artery above the occlusion was measured in the same volunteers. The release rate of t-PA Agfrom the arterial wall was calculated according to the Fickćs law as a product of the difference in t-PA Ag concentration between the occluded and the contralateral artery and the local flow in the brachial artery above the occlusion. t-PA Ag concentration in the occluded artery (3.4 ngžml) was significantly higher than in the contralateral, non occluded artery (2.6 ngžml), p=0.03. During the arterial occlusion PAI-1 Ag concentration did not change significantly. Our results have shown that t-PA is released from the arterial wall and that this release, compared to the published data, is even 4- 8 times more intense than from veins. The arterial wall, on the other side,is not an important source of PAI- 1 in vivo. Confirmation of t-PA release from the arteries stresses its role in the defence against atherosclerosis and arterial thrombosis.