The effect of oral administration, for 24 or 48 hr, of different octadeca fatty acids containing a 9,12-dienoic structure on the fatty acid composition and delta 9 desaturation activity of liver microsomes of rat fed a fat-free diet was studied. The ethyl esters of linoelaidic and gamma-linolenic acids, the methyl ester of linoleic acid and free columbinic acid were administered to rats maintained on a fat-free diet. The supplementation of the fat-free diet with linoelaidate produced no relevant changes in the fatty acid composition pattern of liver microsomes and did not modify the percentage of conversion of palmitic to palmitoleic acid. The addition of linoleate or gamma-linolenate to the fat-free diet returned liver microsome delta 9 desaturation activity toward the control and partially restored the liver microsome fatty acid spectrum found in the fat-free diet. Columbinic acid (5-trans-9-cis,12-cis-18:3), which cannot be transformed into arachidonic acid, also decreased the delta 9 desaturation activity enhanced by the fat-free diet and evoked changes in the microsomal fatty acid composition similar to those produced by the omega 6 fatty acids. These results suggest that the modulation of delta 9 desaturase activity evoked by dietary administration of unsaturated acids of omega 6 series would depend on the cis double bond configuration of these acids.