The oxidation and esterification of 14C labelled eicosapentaenoic acid (20:5,n-3) and arachidonic acid (20:4,n-6) in isolated liver cells has been studied. The bioconversion of C18 and C22 polyunsaturated fatty acids to 20:5(n-3) and 20:4(n-6) is also discussed. Adrenic acid (22:4,n-6) and docosahexaenoic acid (22:6,n-3) are retroconverted to 20:4(n-6) and 20:5(n-3) respectively by peroxisomal beta-oxidation. 20:4(n-6) and 20:5(n-3) are both mainly oxidized in the mitochondria. The peroxisomal contribution to the oxidation of 20:5,n-3 is however larger than with 18:2(n-6) and 18:1(n-9) which are predominantly oxidized by mitochondrial beta-oxidation. Isolated liver cells oxidize more 20:5(n-3) and esterify less than observed with 20:4(n-6) as substrate. In liver cells from essential fatty acid deficient animals 20:5(n-3) and 20:4(n-6) are both efficiently directed to the phospholipids. In hepatocytes from animals fed a diet rich in 18:2(n-6) and 18:3(n-3) arachidonic acid is still to a large extent esterified in the phospholipids while 20:5(n-3) only to a small extent is esterified in this lipid fraction. 18:2(n-6) is much more efficiently esterified in the phospholipids than is 18:3(n-3) which is also rapidly removed from the phospholipids. The delta 4 desaturase activity is increased in essential fatty acid deficiency similar to delta 6 desaturase. The competition between 18:3(n-3) and 18:2(n-6) for delta 6 desaturation in combination with a much higher dietary intake of 18:2(n-6) than of 18:3(n-3) may limit the conversion of dietary 18:3(n-3) to 20:5(n-3).(ABSTRACT TRUNCATED AT 250 WORDS)