The endothelial surface of the vessel wall was studied after various period of recirculation following transitory mechanical hypoxia. The acute changes consisted of partial or total destruction of the endothelium in certain areas. Later on, the regeneration (division and process-formation) of endothelial cells took place over the damaged areas. These events were demonstrated both by transmission and scanning electron microscopy. When severe vessel damage occurred, the subendothelial matrix, collagen and elastic fibres, and the stomata were also set free. Blood cells anchored to these areas were later covered by endothelium. Regeneration was practically complete after 10 days; previously only some small endothelial processes had shown evidence of regenerative activity. In spite of the extensive damage to the endothelium, no occluding thrombus formation was seen. The phagocytes functioned only in removing debris, and the thrombocytes in facilitating endothelial overgrowth.