A single intravenous injection of homocystine led to an increase in the number of circulating endothelial cells in the blood of rats. We observed also increased permeability of the lung capillaries, platelet sequestration and activation of the venostatic thrombosis. Endothelial injury is probably the key mechanism of thrombotic and atherosclerotic complications in homocystinuria, an inborn error of the amino acid metabolism.