Human umbilical vessels obtained from neonates delivered at term after uneventful pregnancies were examined by light and transmission electron microscopy, with the aim of determining whether or not their structure is compatible with possible fluid exchange between the circulating blood and Wharton's jelly. A comparison of arteries and veins showed that although these vessels have common characteristics, they differ in some elements of their fine structure. The endothelium of both vessels appeared to be highly active metabolically. In the artery, the endothelial cells often protruded into the lumen. This aspect was related to the fine filaments concentrated in the basal part of the cells. This zone, free of organelles, was absent in the venous endothelium, but here pinocytotic vesicles and Weibel-Palade bodies were more abundant. The media included the same elements but was much thicker in the arteries than in the veins. There were two cellular types: typical myocytes and myofibroblasts rich in organelles. Their cytoplasmic processes extended into the interstitial space which was occupied by a material with a loose structure, that is, material containing a well-developed ground substance at the expense of the elastic and collagen fibres. The ultrastructural features of the umbilical vessels suggest an increased endothelial permeability, and it is suggested that transfer across the umbilical vessels may play a role in the formation of amniotic fluid.