To determine the essential fatty acid (EFA) requirements of the cat, specific pathogen-free kittens were fed either a linoleate-deficient diet or one of two diets containing 5% safflower seed oil (SSO) with or without 0.2% tuna oil. The diets were fed for 82-101 weeks beginning at 3 months of age. The results showed that linoleate is an essential fatty acid for the cat. Linoleate deficiency resulted in reduced feed efficiency (in males), high rates of transepidermal water loss, poor skin and coat condition, and fatty liver. These manifestations of EFA deficiency were prevented by SSO. Tuna oil had no additional effect. Analyses of the fatty acid composition of plasma, erythrocytes and liver lipids revealed that linoleate deficiency caused changes that were qualitatively, but not quantitatively similar to EFA deficiency in the rat. When SSO was provided, linoleate was elongated and desaturated at the delta 5 position to form 20:2n6 and 20:3(5,11,14). However, there was negligible conversion of linoleate to arachidonate. These results indicate that linoleate has specific functions as an EFA, independent of arachidonate synthesis and prostaglandin formation.