The research of essential fatty acids biochemistry in Argentina was initiated approximately twenty years ago. These investigations together with the outstanding findings obtained in other countries proved the existence of different types of reactions. The essential fatty acids, linolenic and alpha linolenic, are converted by desaturation and elongation reactions in higher homologs building families of fatty acids. These reactions are regulated by dietary components, hormones, temperature and products of reaction. Retroconversion reactions transform the higher homologs, especially of 22 carbons back again to lower homologs avoiding beta oxidation. They are incorporated in the lipids to build the lipoproteic membranes increasing their fluidity. Specific acids are converted to prostaglandins that function as specific hormones.