Fatty acid compositions were determined of phospholipids, isolated from umbilical arteries and veins, obtained from Dutch neonates after vaginal delivery, terminating normal pregnancy. The fatty acid profiles of the cord vessels were characterized by the absence of eicosapentaenoic (timnodonic) acid, a low (2-3%) content of linoleic acid and reasonable amounts of arachidonic acid (10-15%) and docosahexaenoic (cervonic) acid (3-5%). Significant amounts of Mead acid (1-4%) and its direct elongation product (0.5-2%) were also observed. In each cord, the efferent blood vessels contained significantly more Mead acid and other fatty acids of the oleic acid (n-9) family and less fatty acids of the linoleic (n-6) and linolenic (n-3) families than the afferent blood vessel. This indicates that the essential fatty acid (EFA) status of 'downstream' neonatal tissue may be marginal. No signs of EFA-deficiency were observed in endothelial and smooth muscle cells in culture, or in blood vessels from adults. In all cords 22:5(n-6) was significantly higher in the artery compared to the vein, whereas for all other (n-6) fatty acids this difference was negative. Since the synthesis of 22:5(n-6) is known to be stimulated when the required amount of cervonic acid, 22:6(n-3), is too low, our observations also suggest that the cervonic acid status of the neonates investigated was not optimal. Further studies are in progress to relate these findings to maternal EFA status and complications of pregnancy.