Diethylstilbestrol is carcinogenic in rodents and in humans and its peroxidatic oxidation in utero has been associated with its carcinogenic activity. Horseradish peroxidase-catalyzed oxidation of [14C]diethylstilbestrol and [14C]diethylstilbestrol analogs induced binding of radiolabel to DNA only when the compound contained a free hydroxy group (Metzler, M., and Epe, B. (1984) Chem. Biol. Interact. 50, 351-360). We have found that horseradish peroxidase or prostaglandin-H synthase-catalyzed oxidation of diethylstilbestrol in the presence of the spin trap 5,5-dimethyl-1-pyrroline-N-oxide caused the generation of an ESR signal indicative of a free radical intermediate (aN = 14.9 G, aH = 18.3 G). The identity of the trapped radical could not be identified on the basis of published hyperfine coupling constants, but the observation that horseradish peroxidase-catalyzed oxidation of 1-naphthol produced an identical ESR signal suggests that the radical was either a phenoxy or phenoxy-derived radical. During horseradish peroxidase-catalyzed oxidation of diethylstilbestrol in the presence of glutathione the thiol reduced the diethylstilbestrol radical to generate a thiyl radical. This was shown by a thiol-dependent oxygen uptake during horseradish peroxidase-catalyzed oxidation of diethylstilbestrol and the observation of an ESR signal consistent with 5,5-dimethylpyrroline-N-oxide-glutathionyl radical adduct formation. A diethylstilbestrol analog devoid of free hydroxy groups, namely diethylstilbestrol dipropionate, did not produce an ESR signal above control levels during horseradish peroxidase-catalyzed metabolism in the presence of 5,5-dimethylpyrroline-N-oxide. Thus, free radicals are formed during peroxidatic oxidation of diethylstilbestrol and must be considered as possible determinants of the genotoxic activity of this compound.