Changes induced by dietary methyl linoleate and arachidonate in the fatty acid composition of liver phosphatidyl choline, phosphatidyl ethanolamine, cholesterol esters, and triglycerides were investigated in essential fatty aciddeficient rats. The esters were fed for 0, 24, 50, 96, 192, and 360 hr. The lipids were fractionated by thin-layer chromatography and the fatty acid composition was estimated by gas liquid chromatography. Acids bound to the a'- and 8-carbons of phosphatides were separated by lipolysis with phospholipase A. From the compositions found it was deduced that both dietary linoleate and arachidonate inhibited eicosatrienoate synthesis from oleate but that only arachidonate replaced eicosatrienoate quantitatively in the 8-position of lecithin and cephalin. Both dietary acids displaced some of the &positioned oleate. Monoenoic: saturated acid ratios were also decreased by both esters both in triglycerides and in a'-bound acids in phosphatides. In triglycerides this change preceded any significant incorporation of linoleate or arachidonate. Arachidonate effects seemed to be more rapid and more marked than those of linoleate and although final compositions were similar, because of the conversion of linoleate into arachidonate, a different pattern of reactions led to these results.Facultad de Ciencias Médicas