Weanling rats were fed diets that contained 2.5% linoleate (cc-18:2) along with graded levels (1.2–18%) of its trans isomers, either the di-trans isomer (tt-18:2) or a mixture of the mono-trans isomers (ct,tc-18:2). The trans isomers were added at the expense of oleate. After a 6-week feeding experiment produced no significant differences in weight or feed efficiency among the rats, the fatty acid compositions of their depot fats and liver lipids were determined. The depot fats closely resembled the dietary fats, with tt-18:2, tc-18:2, and ct-18:2 being deposited at levels proportional to their levels in the diets. The liver lipids showed a preferential accumulation of cc-18:2, and although tt-18:2 and tc-18:2 were deposited at levels proportional to their dietary concentrations, ct-18:2 did not appear in the liver lipids to an appreciable extent. Apparently it is excluded from the phospholipids which comprised 65% of the liver lipid. Ratios of 18:2 to 20:4 in the liver were also studied. Increasing levels of dietary tt-18:2 caused decreased levels of arachidonate in the liver, suggesting that chain elongation of cc-18:2 is somewhat inhibited by high ratios of tt-18:2 to cc-18:2. This inhibition was appreciable only at 55-18:2/cc-18:2 ratios greatly in excess of the ratios that occur in human diets. The presence of ct,tc-18:2 in the diet did not affect the level of arachidonate in the liver, but did give rise to a trans-20:4 fatty acid, presumably by chain elongation of ct-18:2. Although tc-18:2 serves as an energy source, it is neither a substrate nor an inhibitor for chain elongation.