Estrogens are considered the major breast cancer risk factor, and the carcinogenic potential of estrogens might be attributed to the DNA modification caused by derivatives formed during metabolism. 17β-estradiol (E2), the main steroidal estrogen present in women, is metabolized via two major pathways: formation of the 2-hydroxyestradiol (2-OH E2) and 4-hydroxyestradiol (4-OH E2) through the action of Cytochrome P450 (CYP) 1A1 and 1B1, respectively. Previous reports suggested that 2-OH E2 have putative protective effects, while 4-OH E2 is genotoxic and has potent carcinogenic activity. Thus, the ratio of 2-OH E2/4-OH E2 is a critical determinant of the toxicity of E2 in mammary cells. In the present study, we investigated the effects of the berberine on the expression profile of the estrogen metabolizing enzymes CYP1A1 and CYP1B1 in breast cancer MCF-7 cells. Berberine treatment produced significant induction of both forms at the level of mRNA expression, but with increased doses produced 16~ to 52~fold greater inductions of CYP1A1 mRNA over CYP1B1 mRNA. Furthermore, berberine dramatically increased CYP1A1 protein levels but did not influence CYP1B1 protein levels in MCF-7 cells. In conclusion, we present the first report to show that berberine may provide protection against breast cancer by altering the ratio of CYP1A1/CYP1B1, could redirect E2 metabolism in a more protective pathway in the breast cancer MCF-7 cells.