The metabolism by the fetal rat of [1–14C] linoleic acid (18:2 n-6) was compared with that of [1–14C] α-linolenic acid (18:3 n-3) studied in vivo and in vitro. Both linoleic and α-linolenic acid were rapidly converted to long-chain derivatives in fetal brain but rates of Δ6 desaturation were higher for α-linolenic than for linoleic acid. In the liver α-linolenic, but not linoleic acid, was rapidly converted to long-chain derivatives in vivo. Measurements of Δ6 and Δ5 desaturation by fetal liver in vitro showed significant desaturase activity but this was lower than in the brain. It is suggested that the observed difference in the hepatic metabolism of linoleic compared with that of α-linolenic acid reflects their differing affinities for alternative metabolic pathways.