Three dietary levels of essential fatty acids (EFA); 3.0, 0.75 and 0.07 cal%, and with a linoleic/linolenic acid ratio of 4:1, were fed to rats for more than two generations. The phospholipid concentration and fatty acid composition of lecithin in skeletal muscle were analyzed from term to 120 days of age. Muscle from rats, fed the 3.0- and 0.75-cal% diets, always had a higher concentration of phospholipids than those of rats fed the 0.07-cal% diet. At all ages the concentration of polyenoic acids in the muscle lecithin was influenced by the level of EFA in the diet, but after the age of 45 days, the difference between rats fed 3.0 and 0.75 cal% EFA was very small. The most pronounced influences were found at birth and at weaning of the rats. The sums of fatty acids of the linoleic and linolenic acid series were diminished roughly to the same extent, when rats fed 0.75 cal% were compared to those fed 3.0 cal%, but the levels of fatty acids of the linolenic acid series were more reduced than those of the linoleic acid series in the rats fed 0.07 cal% EFA. Arachidonic acid varied more with the dietary level of EFA than did linoleic acid, while the minor fatty acids of the linoleic acid series were only slightly affected. The deficiency in EFA was to some extent compensated for by the fatty acid 20:3 (n-9), but mainly by monoenoic acids. The saturated fatty acids were not influenced at all.