A method for the preservation of the fine structure of the umbilical vessels, including the endothelium, is described. Electron microscopical observations have been made showing the presence of a metabolically active endothelium; the absence of an internal elastic lamina in the intima; the presence of longitudinally and circularly orientated smooth muscle cells; and well developed vasa vasorum in the media of the artery. The principal differences in the vein were the presence of an attenuated internal elastic lamina in the intima; the presence of circularly orientated smooth muscle cells and longitudinally orientated elastic fibres, and the absence of vasa vasorum.