Maternal and fetal plasma concentrations of free fatty acids, triacylglycerols and phospholipids and the profile of their fatty acids were measured in three catheterized and unanaesthetized sheep. Fetal concentrations of all three lipid fractions were low and did not correlate with maternal concentrations. There were no measurable umbilical venous-arterial differences. Linoleic acid concentrations were low in both mother and fetus. The fatty acid composition of fetal adipose tissue, liver, lung and cerebellum of five animals was analysed. Again linoleic acid levels were very low, but phospholipids contained 2-8% arachidonic acid. [14C] linoleic acid and [3H] palmitic acid were infused intravenously into three ewes. Only trace amounts of labelled fatty acids were found in fetal plasma and these were confined to the free fatty acids. 14C-label was incorporated into fetal tissue lipids, but most of this probably was due to fetal lipid synthesis from [14C] acetate or other water-soluble products of maternal [14C] linoleic acid catabolism. It is concluded that only trace amounts of fatty acids cross the sheep placenta. They are derived mainly from the maternal plasma free fatty acids and might just be sufficient to be the source of the small amounts of essential fatty acids found in the lamb fetus, but are insignificant in terms of energy supply or lipid storage.