Abstract: The chemopreventive potential of two major soy isoflavones, genistein and daidzein, in mammary carcinogenesis remains enigmatic. The aim of the present study was to investigate the chemopreventive potential of orally administered genistein, daidzein and genistein+daidzein in 7,12-dimethylbenz(a)anthracene (DMBA) induced mammary carcinogenesis in Sprague-Dawley rats. The chemopreventive potential was assessed by monitoring the tumor incidence and tumor volume as well as by analyzing the status of biochemical markers (17 β-estradiol (E2), enzymatic and non-enzymatic antioxidants and phase I and phase II detoxification enzymes) during DMBA-induced mammary carcinogenesis. A single subcutaneous injection of DMBA (25 mg rat-1) in the mammary gland developed mammary carcinoma in female Sprague-Dawley rats. Oral administration of genistein (20 mg kg-1 b.wt.), daidzein (20 mg kg-1 b.wt.) and genistein+daidzein (20mg+20 mg kg-1 b.wt.) to DMBA treated rats significantly prevented the tumor incidence and tumor volume as well as brought back the status of above said biochemical variables. Genistein and daidzein in combination have shown pronounced chemopreventive potential than either as genistein or daidzein alone. The present study revealed the chemopreventive potential of genistein+daidzein in combination during DMBA induced mammary carcinogenesis. The chemopreventive potential of genistein+daidzein is probably due to their antilipid peroxidative efficacy and modulatory effect on phase I and phase II detoxification cascade during DMBA induced mammary carcinogenesis.