1. By feeding safflower-seed oil to rats deficient in the essential fatty acids it was found that major changes in the liver and serum triglycerides had occurred in 4 d although the fatty acid composition had not fully returned to normal. 2. Rats which had been on a saturated-fat diet for 18 weeks were given for 4 d, a diet supplemented with safflower-seed oil, methyl linolenate or ethyl arachidonate. Linoleic and linolenic acids failed to reduce the liver triglycerides but had some effect in raising the serum triglycerides to normal. Arachidonic acid reduced liver triglycerides but had no effect on serum lipids. There were marked changes in the fatty acid composition of the phospholipids but little change in the triglycerides. 3. There was good correlation between the concentrations of the phospholipids and the triglycerides in the serum. The concentration of serum phospholipids was positively correlated with the percentage of linoleic and arachidonic acids but negatively correlated with the percentage of palmitoleic, oleic and 5, 8, 11-eicosatrienoic acids. 4. In a further 4 d feeding experiment in which the lipoprotein fraction of very low density from the serum was measured, it was found that safflower-seed oil led to an increase but methyl arachidonate resulted in a decrease in the concentration of the lipids. 5. Extraction of the lipoprotein fraction of very low density from normal and deficient rats with n-heptane at – 18° indicated that phosphatidyl cholines containing stearic acid and either arachidonic or 5, 8, 1 1-eicosatrienoic acid were the most firmly bound. 6. It was concluded that linoleic acid and arachidonic acid had different and specific roles in lipid metabolism.