Diethylstilbestrol-4',4"-quinone (DES Q) has previously been postulated to be a reactive intermediate in diethylstilbestrol (DES) metabolism. DES is oxidized to DES Q in vitro, but the occurrence of the quinone metabolite in vivo has not yet been demonstrated due to its instability and chemical reactivity. In this report, the characteristics of in vitro formation of DES Q and the isolation of 3H-labeled DES Q from tissue extracts of hamsters injected with radiolabeled DES is described. In vitro, the time-dependent formation of DES Q as a function of microsomal protein, cofactor or substrate concentrations was demonstrated. The microsome-mediated oxidation of DES to quinone was inhibited by various compounds that also effectively inhibit the peroxidatic activity of cytochrome P-450. In vivo, the formation of DES Q occurred in all tissues investigated, livers and kidneys of male and female adult hamsters, neonates and fetuses, and in uterus and placenta. Concentrations of quinone metabolite in liver and kidney of adult hamsters after injection of 75 mumol/kg DES were 76 and 20 pmol/g tissue respectively. In neonates and fetus, concentrations of DES Q after the same dose of DES were markedly less than those in adults (0.026 and 0.047% of adult levels in neonatal liver and kidney and 0.013 and 0.016% of adult levels in fetal liver and kidney respectively). Since DES Q was also formed by fetal liver homogenate in vitro, fetal oxidizing enzymes appear to be the source of the quinone metabolite in this tissue. DES Q concentrations were also examined after injection of DES into hamsters pretreated with vitamin C or alpha-naphthoflavone, substances known to inhibit DES-induced renal carcinogenesis. Quinone metabolite levels were cut in half in response to vitamin C in correlation with the approximately 50% decrease in DES-induced renal tumors reported previously. alpha-Naphthoflavone pretreatment decreased renal and hepatic DES Q concentrations by 70 and 17% respectively, also in correlation with the known prevention of kidney tumors by this flavone. These data support a role of DES Q in DES-induced carcinogenesis. Since there is no correlation between DES Q concentrations and target site specificity of DES induced tumors, the oxidation of DES to DES Q and the genotoxicity of this metabolite may be a necessary but not sufficient event in tumor development. Hormone-dependent growth of initiated cells may also be necessary for the occurrence of cancers.