The effect of varying unsaturation degree of dietary lipid on the oxidative response of rat liver microsomes was studied. Three groups of growing male rats were maintained for 6 weeks on 15% fat diets containing either soybean oil, olive oil, or coconut oil, with the same level of vitamin E. After 6 weeks, microsomal malondialdehyde, vitamin E, and fatty acid composition were measured in liver microsomes. The relative abundance of saturated and unsaturated fatty acids in the microsomes reflected the composition of the dietary lipid. When dietary requirement for vitamin E was satisfied, the increased polyunsaturated fatty acid intake from vegetable oils did not enhance lipid peroxidation in physiological conditions, as demonstrated by similar malondialdehyde concentrations found in the three groups. However, the somewhat lower vitamin E content measured in soybean oil-fed rats confirms an enhanced requirement for dietary antioxidant caused by the increased intake of polyunsaturated fatty acids. The susceptibility of liver microsomes to lipid peroxidation stimulated by the ADP/iron/ascorbate system was also studied. Membranes of soybean oil-fed rats exhibited the highest peroxidation rate, as shown by oxygen consumption and malondialdehyde and 4-hydroxy-2,3-trans-nonenal production, because of the lower concentration of vitamin E and of the higher content of polyunsaturated fatty acids. Microsomes of olive oil-and coconut oil-fed rats showed highest protection against lipid peroxidation.