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Background: Lipid peroxidation in tissues and in tissue fractions is a degradative free-radical process that primarily involves polyunsaturated fatty acids; it has been implicated as a major contribution to many types of tissue damage, especially in diabetes melli- tus. This study was designed to investigate the effect of P/S dietary composition on the lipid peroxidation of biomembranes in streptozotocin-induced diabetic rats. Methods: Diabetic Sprague Dawley rats weighing about 200g were randomly divided into 3 groups: Group L(n=8) fed with P/S ratio 0.4, Group M(n=8) fed with P/S ratio 1.2, and Group H(n=8) fed with P/S ratio 2.0. Diabetes was induced by daily intraperitoneal injection of streptozotocin(20mg/kg) for 5 days. After feeding for 6 weeks, plasma cholesterol, HDL-cholesterol, RBC membrane peroxidation and hepatic mitochondrial lipid peroxidation were measured. Results: Total cholesterol(mmol/L) was decreased and HDL-cholesterol/total cholesterol % ratio was increased by increment of P/S ratio in a doseindependent manner(L 3.4+-0.32 and 8.7; M 2.5+- 0.29 and 12.6; H 2.5+-0.29 and 12.3). RBC mem- brane lipid peroxidation(nmol/mI. packed RBC) was higher in H(3.21+-0.20) than that in L(2.02+-0.19) or M(2.40+-0.21) (p<0.05). Hepatic mitoehondrial lipid peroxidation (nmol/g protein) was lower in L(7.5+- 1.25) than that in M(11.7+1.57) or H(14.0+2.04) (p<0,05). Conclusion: P/S dietary increment increased the lipid peroxidation of biomembranes(RBC membrane an4 hepatic mitochondrial membrane) in streptozotocin-induced diabetic rats.