Scanning electron microscopy of the rabbit thoracic aorta following staining with silver nitrate showed endothelial cell borders to be marked by zigzag lines outlining individual endothelial cells as oblong rhombic fields oriented with the long axis parallel to the aortic axis. The layer of endothelial cells coated an underlying layer of coarse longitudinal folds, probably foldings of the underlying elastic membranes. The individual endothelial cells were inconspicuous in uncoated specimens. Double coating with coal and gold revealed a grainy porous mass in the silver lines and a flat surface of endothelial cells studded with small round elevations. Comparative studies with transmission light microscopy and transmission electron microscopy indicated silver deposits in the endothelial cell boundaries. Fine dustlike silver sediments were also observed on the surface of endothelial cells and in the layers next to the intima.