It is now established that fatty acid 7,10,13,16-22:4 is metabolized into 4,7,10,13,16-22:5 as follows: 7,10,13,16-22:4!9,12,15,18- 24:4!6,9,12,15,18-24:5!4,7,10,13,16-22:5. Neither C#% fatty acid was esteri®ed to 1-acyl-sn-glycero-3-phosphocholine (1-acyl- GPC) by microsomes, whereas the rates of esteri®cation of 4,7,10,13,16-22:5, 7,10,13,16-22:4 and 5,8,11,14-20:4 were respectively 135, 18 and 160 nmol}min per mg of microsomal protein. About four times as much acid-soluble radioactivity was produced when peroxisomes were incubated with [3-"%C]- 9,12,15,18-24:4 compared with 6,9,12,15,18-24:5. Only [1-"%C]7,10,13,16-22:4 accumulated when [3-"%C]9,12,15,18-24:4 was the substrate, but both 4,7,10,13,16-22:5 and 2-trans- 4,7,10,13,16-22:6 were produced from [3-"%C]6,9,12,15,18-24:5. When the two C#% fatty acids were incubated with peroxisomes, microsomes and 1-acyl-GPC there was a decrease in the production of acid-soluble radioactivity from [3-"%C]6,9,12,15,18-24:5, but not from [3-"%C]9,12,15,18-24:4. The preferential fate of [1-"%C]4,7,10,13,16-22:5, when it was produced, was to move out of peroxisomes for esteri®cation into the acceptor, whereas only small amounts of 7,10,13,16-22:4 were esteri®ed. By using #H-labelled 9,12,15,18-24:4 it was shown that, when 7,10,13,16- 22:4 was produced, its primary metabolic fate was degradation to yield esteri®ed arachidonate. Collectively, the results show that an inverse relationship exists between rates of peroxisomal b-oxidation and of esteri®cation into 1-acyl-GPC by microsomes. Most importantly, when a fatty acid is produced with its ®rst double bond at position 4, it preferentially moves out of peroxisomes for esteri®cation to 1-acyl-GPC by microsomes, rather than being degraded further via a cycle of b-oxidation that requires NADPH-dependent 2,4-dienoyl-CoA reductase.