Microsomal fatty acid desaturation is defective in streptozotocin-induced experimental diabetes. This defect is correctable by insulin treatment. The electron transport chain needed for microsomal fatty acid desaturation was studied in liver microsomes of streptozotocin diabetic rats and the defect was localized to the terminal desaturase enzyme. Cytochrome b5 levels were elevated in the face of decreased fatty acid desaturation and returned to normal after 48 h of insulin treatment; 2 U [units] of regular insulin every 6 h for 24 h repaired the fatty acid desaturation defect, while 0.5 U failed to correct the defect. Both the .DELTA.6 and .DELTA.9 desaturase defects (linoleic acid and stearoyl-CoA desaturation) required similar amounts of insulin and periods of time for correction, although these are different enzymes. This is consistent with the desaturation defect being due to a protein synthetic effect. Diabetic rats treated twice daily with injections of 4 U of NPH [neutral protamine Hagedorn] insulin showed a super repair of their desaturase defect by 48 h: .DELTA.9 desaturase activity increased 8 times over control activity, while .DELTA.6 desaturase activity increased 2 1/2 times over control activity. This, together with the fact that .DELTA.6 desaturase activity in diabetes (64% of control) is altered less than is .DELTA.9 desaturase activity (22% of control), indicates that .DELTA.6 desaturase enzyme activity is less responsive to insulin than is .DELTA.9 desaturase enzyme activity. The physiologic significance of altered fatty acid desaturation in diabetes mellitus is unknown.