The desaturation, chain elongation and esterification of [1-14C]eicosapentaenoic acid, [1-14C]arachidonic acid, [1-14C]eicosatrienoic acid, [1-14C]linolenic acid and [1-14C]linoleic acid were studied in isolated liver cells. Rats fed diets with either 15% hydrogenated coconut oil or 15% partially hydrogenated marine oil, both deficient in essential fatty acids, 15% soybean oil or standard pellet diet with 6% fat, were used. The delta 4-desaturation of 22:5(n - 3) and 22:4(n - 6) as well as the delta 6-desaturase activity was distinctly higher in cells from animals fed coconut or marine oil than with soybean oil or standard pellet. The rate of delta 5-desaturation of 20:3(n - 6) and 20:4(n - 3) was nearly the same in cells from rats fed coconut, marine and soybean oils and higher than with standard pellet. The chain elongation of 20:5(n - 3) to 22:5(n - 3) was distinctly more pronounced than the elongation of 20:4(n - 6) with all four diets. 20:5(n - 3) was mainly esterified in the phospholipids with marine and coconut oils, and mainly in triacylglycerol with standard pellet and soybean oils. The proportion of [1-14C]20:4(n - 6) in the phospholipids to that in triacylglycerol decreased in the order marine oil greater than coconut oil greater than standard pellet greater than soybean oil. The different endogenous arachidonic acid content in the phospholipids induced by the different diets increased in the same order. 20:5(n - 3) was rapidly esterified in triacylglycerol and phospholipids, then liberated especially from the triacylglycerol fraction, chain elongated to 22:5(n - 3) and reesterified.