MOST vertebrate species require some dietary source of essential fatty acids (EFAs)1,2. A wide range of naturally occurring polyunsaturated fatty acids have been shown to exhibit EFA activity, but they can be classified into two homologous series characterised by the position of the terminal double bonds relative to the methyl (omega) carbon atom. The ω6 series of EFAs all have double bonds in the ω6 and ω9 positions, the ω3 series in the ω3, ω6 and ω9 positions. The metabolic interrelationships of the members of each series are shown in Fig. 1. The naturally occurring 18-carbon parent compounds for each series are linoleic acid (18:2 ω6) and linolenic acid (18:3 ω3) and EFA requirements are usually stated in terms of either or both ot these parent EFAs (p-EFAs)1,3.