The incorporation of [1-14C]-linoleic acid and [1-14C]-arachidonic acid into the lipids of ovine placental homogenate was studied. Both fatty acids were incorporated mainly into the phospholipid fraction with much lower levels being found in the diacylglycerols and triacylglycerols, particularly when the substrate was [1-14C]-arachidonic acid. Within the phospholipid fraction, both substrates were incorporated largely into phosphatidic acid although significant levels of incorporation were found in the phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol fractions. The incorporation of [1-14C]-arachidonic acid into phosphatidylcholine was lower and into phosphatidylethanolamine and phosphatidylinositol was higher than those levels found when the substrate used was [1-14C]-linoleic acid. While some of the trienoic and tetraenoic, but not pentaenoic, desaturation products of [1-14C]-linoleic acid were detected in phosphatidylcholine and phosphatidylethanolamine, the highest levels of incorporation of these metabolites were into phosphatidic acid. Similarly, the pentaenoic products of [1-14C]-arachidonic acid were found mainly in phosphatidic acid although significantly higher levels were observed in phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol. The results suggest that in ovine placental tissue polyunsaturated fatty acids, whether supplied pre-formed or made available by desaturation and elongation processes, could potentially be incorporated into phospholipids by the de novo route via phosphatidic acid.