Incorporation of [10-14C] oleic acid or [10-,4C] elaidic acid into tissue lipids by rats was studied 4h after the intubation of labelled compounds. The animals were prefed 10% fat diets containing either corn oil (CO) or hydrogenated coconut oil (HCO) for 16weeks. In both dietary groups, animals deposited a significantly higher amount of 14C-elaidic acid than 14C-oleic acid in tissue lipids of liver, plasma and adrenal glands. Separation of liver lipids showed that oleic acid was incorporated preferentially into the triglyceride fraction. Elaidic acid, however, was incorporated predominantly into the phospholipid fraction. In the CO-fed rats, chylomicrons, low density lipoprotein, and high density lipoprotein in the plasma were labelled to a much greater extent in the 14C-elaidate-treated animals. This effect was less apparent when animals were fed hydrogenpted coconut oil. Fractionation of liver fatty acids showed that the predominant portion of the recovered radioactivities remained in the octadecenoic acid fraction. In the EFA-deficient rats, with intubation of 14C-oleic acids, 3.4% of the total radioactivity was associated with eicosatrienoic acid; with intubation of 14C-elaidic acid, on the other hand, incorporation of 1 ‘C label into this acid was negligible. In all experimental conditions, radioactivity found in the 16 and 18-carbon saturated fatty acids suggests that these acids can be formed from oleic and elaidic acid via hydrogénation. However, the quantity of these in vivo conversions was very small.