This study aimed to investigate the role of the phytoestrogen (chrysin) in the treatment or protection of NMU-induced preneoplastic lesions in mammary glands of postmenopausal female rats. Rats (n=105) were divided into seven groups (n=15/group); group1(negative control group).Groups 2&3 (positive control groups), rats were injected intraperitoneally (i.p.) with 4 doses of NMU (75 mg/kg b.w.,once/5days) before or after oral administration of glycofurol. Groups 4 &5(treatment groups),rats received 4 doses of NMU (75 mg/kg b.w., once/5days,i. p.), left for 6 weeks then followed by 3 doses, per week, of chrysin (125 or 250 mg/kg b.w., orally; p.o), respectively, for another 6 weeks. Groups 6 & 7(protective groups), rats received 3 doses, per week, of chrysin(125 or 250 mg/kg b.w.,p.o.), for 6 weeks then followed by 4 doses of NMU (75 mg/kg b.w., once/5days, i.p.) then left for another 6 weeks. The NMU injection caused significant decrease in PLT count, lymphocytes %, Hb, MCH, MCHC, E₂, TAC levels as well as GST and CAT activities. The results also showed that NMU significantly elevated WBC count, granulocytes%, monocytes %, MCV, RDW%, CRP, CEA levels and arginase activity. Either treatment or protection with chrysin modulated the adverse effects of NMU and ameliorated the change in the biochemical parameters. The biochemical observations were also confirmed by histological studies. In conclusion, phytoestrogens may relieve the severity of postmenopausal pre-cancerous disorders, especially when consumed in high dose for a while before the incidence of lesions.