Rat liver microsomal delta 6 and delta 5 desaturation are defective in experimental diabetes, but this defect is correctable with insulin treatment. Rat liver fatty acid composition and delta 6 and delta 5 desaturation were studied in the spontaneously diabetic adult female Bio-Breeding (BB) rat. Control Wistar rats and BB rats (4 weeks of diabetes), that received insulin (1 IU.100 g body weight-1.day-1), were killed 20 h after the last insulin injection. delta 6 and delta 5 desaturase activities were estimated from the incubation of liver microsomes with (1-14C) 18:2, n-6 or (2-14C) 20:3, n-6, respectively, and the fatty acid composition of the liver and microsomal liver lipids were investigated. Under experimental conditions delta 6 and delta 5 desaturase activities were unchanged in the BB rats when compared to the control rats. Impairment of the liver fatty acid composition of diabetic BB rats is not consistent with normal desaturase activity and may be explained by factors other than desaturation disturbance.