Light microscopy, scanning electron microscopy, and transmission electron microscopy were employed on the anterior wall of the rabbit thoracic aorta with the particular purpose to study endothelial marginal flaps and the structure of stomata under physiological conditions at 100 mm Hg. Intra-aortic pressure monitoring was performed during the whole period of perfusion (2 h), restricting pressure variations to a maximum of ± 5 mm Hg in order to preserve endothelial architecture. We found the endothelial surface uniformly smooth. Marginal flaps of the endothelial cells were mutually overlapping, often forming multiple layers. Underlying flaps might protrude to the lumen between neighbouring flaps as flat mushrooms. Flaps of adjacent cells might diverge for short distances to decover an underlying endothelial cell. Downstream flaps of the cells might be overlapped at their base by flaps of adjacent cells, the extremity of the flap thus appearing as a disc separated from the cell. The small discs and troughs thus formed are considered equivalent to the so-called stomata