We studied liver microsomal delta 9 stearic acid desaturase activity and fatty acid composition of liver phospholipids and microsomal total lipids in the insulin-dependent spontaneously diabetic adult male Bio-Breeding rat. The diabetic Bio-Breeding rats (3 weeks of diabetes) were killed 48, 17 and 3 h after the last insulin injection (1.0 IU, 100 g body weight-1 day-1). Under these experimental conditions, delta 9 desaturase activity was defective during the normo- and hyper-glycaemic periods and restored during the hypoglycaemic period which followed the insulin injection to the diabetic rats. The fatty acid composition of diabetic rat liver phospholipids and microsomal total lipids were not consistent with delta 9 desaturase activity at the different periods of glycaemia and may be explained by factors other than disturbances of this desaturation.