In the present study, the antioxidant effects of estradiol (E2) and 2-hydroxyestradiol (2-OHE2) on microsomal lipid peroxidation induced by Fe3+/ADP/NADPH and Fe2+/ascorbate are described. The extent of lipid peroxidation was measured by thiobarbituric acid reactive substances (TBARS) detection, low-level chemiluminescence, and oxygen consumption. 2-OHE2 had a potent antioxidant activity, which in all cases was higher than that of E2. In the Fe2+/ascorbate model, 2-OHE2 showed a similar pattern of inhibition, irrespective of the presence of NADPH or the functionality of microsomes. However, E2 produced only a slight inhibition when either denatured microsomes or native microsomes without NADPH were used, whereas its protective effect increased considerably when microsomal E2 metabolism was favored. During enzymic Fe3+/ADP/NADPH-induced lipid peroxidation, both E2 and 2-OHE2 were found to provide good protection. Results underline the importance of the chemical structure of these compounds and the role of estradiol metabolism in its antioxidant effects.