Studies suggested that in human adults, linoleic acid (LA) inhibits the biosynthesis of n-3 long-chain polyunsaturated fatty acids (LC-PUFA), but their effects in growing subjects are largely unknown. We used growing pigs as a model to investigate whether high LA intake affects the conversion of n-3 LC-PUFA by determining fatty acid composition and mRNA levels of Δ5- and Δ6 desaturase and elongase 2 and -5 in liver and brain. In a 2 × 2 factorial arrangement, 32 gilts from eight litters were assigned to one of the four dietary treatments, varying in LA and α-linolenic acid (ALA) intakes. Low ALA and LA intakes were 0.15 and 1.31, and high ALA and LA intakes were 1.48 and 2.65 g/kg BW0.75 per day, respectively. LA intake increased arachidonic acid (ARA) in liver. ALA intake increased eicosapentaenoic acid (EPA) concentrations, but decreased docosahexaenoic acid (DHA) (all P < 0.01) in liver. Competition between the n-3 and n-6 LC-PUFA biosynthetic pathways was evidenced by reductions of ARA (>40%) at high ALA intakes. Concentration of EPA (>35%) and DHA (>20%) was decreased by high LA intake (all P < 0.001). Liver mRNA levels of Δ5- and Δ6 desaturase were increased by LA, and that of elongase 2 by both ALA and LA intakes. In contrast, brain DHA was virtually unaffected by dietary LA and ALA. Generally, dietary LA inhibited the biosynthesis of n-3 LC-PUFA in liver. ALA strongly affects the conversion of both hepatic n-3 and n-6 LC-PUFA. DHA levels in brain were irresponsive to these diets. Apart from Δ6 desaturase, elongase 2 may be a rate-limiting enzyme in the formation of DHA.