In animal and human tissues, there are four families of fatty acids that are derived from the precursors palmitoleic, oleic, linoleic, and linolenic acids. Of these, linoleic and linolenic acids are essential dietary elements for humans and all higher animals. The four precursor fatty acids are metabolized (through desaturation and chain elongation) to form the long-chain polyunsaturated fatty acids. The principal fatty acids derived from linoleic acid are arachidonic acid, which contains four double bonds (tetraene) and dihomogamma linolenic acid, whereas products formed from linolenic acid are eicosapentaenoic acid and docosahexaenoic acid. The major products from palmitoleic and oleic acids are the 20 carbon fatty acids with three double bonds (triene). In the presence of adequate dietary amounts of essential fatty acids, tetraene products predominate in plasma. When the intakes of both linoleic and linolenic acids are low, triene formation is high and hence the triene/tetraene ratio in plasma is used to assess the index of essential fatty acid deficiency. The polyunsaturated fatty acids derived from essential fatty acids serve as cellular membrane phospholipid components which can influence the physicochemical characteristics of the lipid bilayer. Changes in membrane lipids can modify the mobility and function of a variety of membrane proteins which may result in altered cell/organ functions. Linoleic acid is specifically required in the skin to maintain the integrity of epidermal water barrier. The dihomogamma linolenic acid, arachidonic acid, and eicosapentaenoic acid are the precursors of eicosanoids which influence many cell processes and organ functions.(ABSTRACT TRUNCATED AT 250 WORDS)