A method for estimating fibrinolytic activity of the tissue sections has been worked out. This method consists in covering tissue sections with thin fibrin layers and in microscopic evaluation of their digestion. Using two types of fibrin layers: a) prepared from fibrinogen containing plasminogen, b) prepared from plasminogen free fibrinogen, it is possible to differentiate, whether the digestion is due to the plasminogen activator or to the active enzyme (plasmin). Fibrinolytic activity of the section of dog kidney tissue has been investigated. Experiments were performed on normal kidneys and on kidneys damaged by the mercury chloride intoxication. Studying normal dog kidney it has been found that plasminogen activator is present in juxtaglomerular cells, macula densa and Goormaghtigh cells. It appears that the endothelial cells of the glomerular vessels contain, besides plasminogen activator, an active enzyme, probably plasmin. In the medullary portion of the section of normal kidney tissue no fibrinolytic activity has been found. In dogs intoxicated with mercury chloride the plasminogen activator decreases in vascular endothelia but it completely disappears in the juxtaglomerular apparatus. The injury of the juxtaglomerular apparatus due to mercury chloride intoxication has been also found by routine histopatho-logical investigations. It is suggested that the plasminogen activator, secreted by the kidney into the general circulation, orginates mainly in the juxtaglomerular cells. * Aided in part by the Polish Academy of Sciences, Department II, Biochemical and Biophysical Committee.