It has been demonstrated that estrogens are potent antioxidants and protect against H2O2-mediated depletion of intracellular ATP in human lens epithelial cells (HLE-B3) [Invest. Ophthalmol. Vis. Sci. 44 (2003) 2067]. To investigate the mechanism by which 17beta-estradiol (17beta-E2) protects against oxidative stress, HLE-B3 cells were exposed to insult with H2O2 at physiological (50 microm) and moderately supra- physiological (100 microm) levels over a time course of several hours, with and without pretreatment with 17beta-E2. The ability of 17beta-E2 to prevent H2O2-induced injury to several oxidant susceptible components of the cellular ATP generating machinery, including abundances of mitochondrial gene transcripts encoding respiratory chain subunits and cytochrome c, the glycolytic pathway enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and the energy-shuttling creatine kinase (CK) system, and mitochondrial membrane potential (deltapsi(m)) a measure of mitochondrial membrane integrity, were determined 3 hr after oxidative insult. Northern blot analysis revealed H2O2-induced reductions in mitochondrial transcripts for nicotinamide adenine dinucleotide dehydrogenase (NADH) subunits 4 and 5 and cytochrome c. H2O2 also inactivated GAPDH but did not alter CK activity. Pretreatment and simultaneous addition of 17beta-E2 with H2O2 did not prevent the reductions in mitochondrial transcript levels and GAPDH activity. 17beta-Estradiol did moderate the collapse of mitochondrial membrane potential (deltapsi(m)) in response to H2O2 as demonstrated by JC-1 staining and fluorescence microscopy. Although the precise mode of action responsible for protection by estradiols against oxidative stress remains to be determined, these results indicate that the hormone stabilizes the mitochondrial membrane, thereby preserving the driving force for oxidative ATP synthesis.