Two endogenous antioxidants that are speculated to be defense substances against preeclampsia, 2-hydroxyestradiol (2-OH-E2) and its 17-sulfate, 2-hydroxyestradiol 17-sulfate (2-OH-E2-17-S), were administered to rats to compare their inhibitory effects on hepatic microsomal lipid peroxidation, and the lipid peroxides were determined in NADPH- and ascorbic acid-dependent systems. The two catechols showed a strong inhibitory effect on lipid peroxidation in both systems, and the effect was dose dependent. However, a large difference was observed in their inhibition patterns. After administration of 2-OH-E2, the effect appeared immediately and decreased gradually with time. In contrast, the effect of 2-OH-E2-17-S appeared some time after administration and persisted for a longer time. Both catechols also showed a striking difference in their dynamics. After administration, 2-OH-E2 was detected in the blood together with its metabolites, 2-methoxyestradiol and 2-methoxyestrone, and they disappeared immediately. In contrast, 2-OH-E2-17-S was present in the blood for a longer time together with its O-methylated product, 2-methoxyestradiol 17-sulfate, but disappeared from liver microsomes within 2 h after administration. The results imply no occurrence of a direct inhibition effect of 2-OH-E2-17-S.