We demonstrate for the first time that diethylstilbestrol (DES), a synthetic estrogen, is converted by nuclei to histone‐binding metabolite(s). Reaction of [3H]DES with nuclei in the presence of cumene hydroperoxide or NADPH revealed binding of [3H]DES to histone nuclear proteins. Cel electrophoresis experiments revealed that all five histones, 1, 2A, 2B, 3, and 4, were irreversibly bound to [3H]DES. Histones 1 and 3 were more susceptible to the attack by [3H]DES quinone, a metabolite of DES, than histones 2A, 2B, or 4. The kinetic constants,KmandVmax, of this binding reaction in the presence of cumene hydroperoxide were 10 μM and 750 pmol/mg protein/30 min, respectively. This binding was significantly inhibited by cytochromes P‐450 inhibitors. Low‐molecular‐weight thiols, such as glu‐tathione and cysteine, or thiol modifiers, such as n‐ethylmaleimide, dithionitrobenzoic acid, and hydroxymercuric benzoate, drastically inhibited binding of [3H]DES quinone to histone 3. The binding of [3H]DES metabolites to both transcriptionally active and inactive chromatin histone proteins was observed. We conclude that DES is metabolized to histone‐binding metabolites, presumably by nuclear cytochromes P‐450. DES quinone may be one of the histone‐binding DES metabolites. These data suggest that an analogous in vivo modification in the transcriptionally active chromatin histones by DES metabolites may influence gene function.