Male rats fed a 9% casein-purified diet for 14 days were compared with a similar group of rats whose diet contained 0.25% DL-ethionine. At the end of the feeding period, the animals were tube fed their respective diets plus 1-14C-linoleic acid, and control and experimental rats randomly chosen were then killed 18 h and 30 h thereafter. Composition and radioactivity were determined in carcass and liver lipids. Total amounts of carcass and liver lipids did not differ among the groups, nor were there any differences in the distribution or specific activity of the carcass linoleate. Liver triglyceride of rats fed ethionine, while being no higher in concentration than that of control animals, did, however, have a higher proportion of radioactivity and more linoleic acid with a higher specific activity than did the controls. The differences were greatest at the 18-h post-feeding period. Ethionine also increased the proportion of linoleic acid and decreased that of arachidonic acid in the liver phospholipids as compared with controls. Analysis of the phosphatidylethanolamines and phosphatidylcholines showed that by the 18-h post-feeding period, the proportions of radioactivity and specific activity of arachidonic acid in both phospholipid fractions were reduced far below control values, whereas the specific activity of linoleic acid was high and similar for all groups. However, by 30 h after administration of the labeled linoleic acid, specific activities, but not the distribution of the label, had returned to normal ranges. From the results it was concluded that the liver phospholipid fatty acid patterns seen in rats fed ethionine were a consequence of an impairment by the analogue in the normal utilization and conversion of linoleic acid to arachidonic acid.