Twenty necroptic atherosclerotic aortas were studied using a modified en face Hautchen preparation method. Endothelium of 1 x 1 cm pieces of aortas was frozen onto slides with the help of dry ice. Endothelial cells on atherosclerotic lesions were irregularly oriented, had ununiform shape and giant multinucleated endothelial cells were present. The endothelial pavement on atheromatous lesions was usually defective. There was increased activity of ATPase and 5'Nase in endothelial cells on atherosclerotic lesions and on their borders. It is suggested that the high activity of ectonucleotidases of the endothelium on atherosclerotic lesions results in rapid dephosphorylation of ATP and ADP released by platelets aggregating to adenosine. Adenosine accumulated in the unstirred layer of plasma in the macrovasculature can effectively inhibit platelet aggregation and subsequent thrombosis on pathologically changed vascular pavement in atherosclerosis.