We determined the changes in the mutagenic and estrogenic activities of 17 beta-estradiol after a nitrite treatment. Nitrite-treated 17 beta-estradiol showed mutagenic activities toward Salmonella typhimurium strains TA 100 and TA 98. We confirmed that nitrite-treated 17 beta-estradiol generated radicals from the results of an analysis of electron spin resonance. By applying an instrumental analysis, we identified 2-nitro-17 beta-estradiol to have been formed in the reaction mixture. 2-Nitro-17 beta-estradiol did not exhibit mutagenic activities toward Salmonella typhimurium strains, suggesting that other mutagens might have been formed in the reaction mixture. The clastogenic properties of nitrite-treated 17 beta-estradiol and 2-nitro-17 beta-estradiol were analyzed by a micronucleus test with male ICR mice. Nitrite-treated 17 beta-estradiol and 2-nitro-17 beta-estradiol induced a significantly higher frequency of micronucleated reticulocytes in mice. The estrogenic activity of 2-nitro-17 beta-estradiol was found to be lower than that of 17 beta-estradiol. These data suggest that a daily oral intake of 17 beta-estradiol and nitrite might induce the formation of mutagenic compounds in our body.