Fatty acid compositions of phospholipids of heart, liver, kidney, aorta, and serum from rats having streptozotocin-induced diabetes were determined and compared with those of nondiabetic controls. Linoleic and dihomo-γ-linolenic acids were increased whereas arachidonic acid was decreased in most tissues, suggesting an impairment of Δ5-desaturase activity. Acids derived from linolenic acid were increased in some diabetic tissues from diabetic animals although the linolenic content was normal, indicating less impairment in the desaturation of the ω3 series of fatty acids. Diabetes suppressed all polyunsaturated acids in the whole animal, but the competition between ω3 and ω6 acids favored the excessive suppression of long-chain ω6 acids and an increase in the proportion of ω3 acids in lipids of vital tissues. These changes in fatty acid composition of the phospholipids may have significant effects on cellular functions and vasoregulatory control mechanisms in diabetes.