The fatty acid composition of plasma phospholipids, triglycerides, cholesterol esters and nonesterified fatty acids was determined by high-resolution capillary gas-liquid chromatography in 41 pairs of mothers and their term infants at time of birth. The total free fatty acid content in maternal and cord plasma was positively correlated, possibly reflecting a passive, gradient dependent transplacental passage of nonesterified fatty acids. Higher percentage values of several saturated and monounsaturated fatty acids in cord than in maternal plasma phospholipids, triglycerides and nonesterified fatty acids may have resulted from an active fetal fatty acid synthesis. Trans fatty acids were found in every lipid class at similar or slightly lower percentages in neonatal as in maternal plasma, thus confirming their placental passage. Long-chain n-6 and n-3 polyunsaturated fatty acids are preferentially incorporated into phospholipids and sterolesters of both maternal and cord plasma. Linoleic and alpha-linolenic acids were found in smaller portions in cord than in maternal fatty acids, in contrast to strikingly higher proportions of their long-chain polyunsaturated metabolites, which may indicate a discriminating placental transport for certain physiologically important long-chain polyunsaturated fatty acids. The fetus appears to obtain fatty acids from a combination of de novo synthesis, a passive gradient dependent transplacental passage of nonesterified fatty acids and a selective materno-fetal placental transport for certain fatty acids, such as physiologically important long-chain polyunsaturated fatty acids.