Cats fed a diet containing linoleate as the only polyunsaturated fatty acid showed extremely low levels of arachidonate in the plasma lipids, as well as an increase in linoleate, eicosadienoate and an unknown fatty acid. Administration of [1-14C]linoleic acid and [2-14C]eicosa-8,11,14-trienoic acid to cats showed that in the liver there was no conversion of the [1-14C] 18:2 to arachidonate, whereas there was significant metabolism of [2-14C] 20:3 to arachidonate. It was found when methyl-gamma-linolenate was fed to cats that the level of 20:3 omega 6 and 20:4 omega 6 in the erythrocytes increased significantly. These results show that there is no significant delta 6 desaturase activity in the cat, whereas chain elongation and delta 5 desaturase enzymes are operative. The unknown fatty acid was isolated from the liver lipids and shown to be a 20-carbon fatty acid with 3 double bonds and which by gas liquid chromatography could be separated from 20:3 omega 9 and 20:3 omega 6. The presence of the delta 5-desaturase activity and the results of the ozonolysis studies indicated that this unknown fatty acid was eicosa-5,11,14-trienoic acid.