1. The syndrome induced by depriving cats of animal lipid is partially cured by feeding 18:3ω6. This is associated with an increase in levels of 20:3ω6, but not 20:4ω6, in plasma phospholipids.
2. It is concluded that the cat lacks Δ5 desaturase activity and has a dietary requirement for 18:3ω6 and possibly 20:4ω6.