Natural and synthetic estrogens elicit normal hormonal responses in concentrations in a clearly defined yet low range. At elevated doses, metabolic reactions of the phenolic moiety, while harmless at low levels, may become the predominant biochemical activity and may exert deleterious effects. These metabolic pathways, such as i) oxidation of estrogens to catechol estrogens and further to their respective quinones, and ii) free radical generation by redox cycling between catechol estrogens or diethylstilbestrol and their quinones, are investigated for their influence in physiological or pathophysiological processes. In this review, the in vitro capacity of various enzymes to oxidize estrogen hydroquinones to quinones or to reduce corresponding quinones to hydroquinones is evaluated. The in vivo activities of enzymes supporting redox cycling of estrogens and free radical generation is correlated with induction of kidney tumors in Syrian hamsters. Concomitant changes in activities in quinone reductase and other detoxifying enzymes in kidneys of hamsters treated with estrogen support a role of free radicals in the induction of tumors by estrogen. Free radical damage to protein and possibly to DNA in kidneys of estrogen-treated hamsters may be used as markers of free radical action in vivo.