In order to estimate the linoleic acid requirement of the rat, four groups of weanling male Wistar-Rats, 18 animals each, were fed isoenergetic semi-synthetic diets containing 14 cal% fat. The linoleic acid content (as linoleic acid methyl ester) amounted to 0; 0.5; 1.3 and 4.0% of total energy intake. The experiment lasted 14 weeks. The parameters analysed were the concentrations of total lipids (TL), free cholesterol (Ch), cholesterol esters (ChE), triglycerides (TG) and phospholipids (PL) in plasma and liver. - Clinical signs of linoleic acid deficiency were only found in the rats fed the linoleic acid free diet. With 0.5 cal% linoleic acid in the diet no deficiency symptoms were observed. - In plasma of the linoleic acid deficient animals the concentrations of TL, Ch, ChE and TG were decreased. Plasma PL contents were insignificantly altered. - While the contents of TL, TG and ChE in the liver of the deficient rats increased significantly, those of PL and Ch were hardly affected. The results show that a linoleic acid supply of 0.5 cal% prevents nearly all alterations of plasma and liver lipid concentrations noticed in linoleic acid deficiency. The effect of this dose was as good as that of 1.3 cal% linoleic acid. Therefore we assume that the minimum requirement of male young rats for linoleic acid is markedly less than 1,3% of total energy intake.