Objective     To observe the effect of n-6/n-3 polyunsaturated fatty acid dietary of different constituent ratio on the serum level of estradiol and expressions of CYP17

CYP19 in MNU-induced rat breast cancer tissues.      Methods     Totally 65 female SD rats were randomly divided into 5 groups: n-6 PUFA

10∶1  n-6/n-3 PUFA

5∶1  n-6/n-3 PUFA

1∶1  n-6/n-3 PUFA and normal control. Except control group receiving physiological saline

all rats were given a single intraperitoneal injection of MNU (50 mg/kg) and switched to different diets containing fatty acids mentioned above. After 8 and 18 weeks

the serum level of estradiol and expressions of CYP17

CYP19 in the breast tissues of different groups were determined by RIA and RT-PCR.      Results     The serum level of estradiol and expressions of CYP17

CYP19 in rat breast cancer tissues were higher than normal control

highest in n-6 group and gradually decreased with the ratio of n-6/n-3 decreasing.       Conclusion     n-6/n-3 PUFA dietary of different constituent ratio can diversely influence the serum level of estradiol and expressions of CYP17

CYP19 in MNU-induced rat breast cancer tissues. n-6/n-3 PUFA dietary with constituent ratio of 1∶1 can significantly inhibit the increased serum level of estradiol and increased expressions of CYP17

CYP19 in breast cancer tissues.