Objective To investigate whether reproductive lactation can reduce the probability of dimethylbenz anthrancence (DMBA)induced breast cancer in female SD rats.Methods Forty 8-week-old female SD rats were divided into two groups by using random number table method,20 in the unfertilized control group and 20 in the unfertilized DMBA group.Eighty 36-week-old female SD rats were randomly divided into fertile and non-lactating control group,fertile and non-lactating DMBA group,fertile and lactating control group,and fertile and lactating DMBA group with 20 rats each.The sterile DMBA group,fertile non-lactating DMBA group and fertile lactating DMBA group were given 10 mg/100 g DMBA by gavage (the interval between the two doses was 2 weeks).Non-fertility control group,childbearing non-lactation control group and childbearing and lactation control group were given the same dose of normal saline by gavage as parallel control.After the administration of DMBA for 24 weeks,the experimental group and the corresponding control group were all sacrificed,and the mammary glands were dissected to observe,stained and counted with argyrophilic nucleolar organizer region (AgNOR),and the expressions of proliferating cell nuclear antigen (PCNA),Ki-67,human epidermal growth factor receptor (C-erb B-2),minichromosome maintenance proteins (MCM)2 in rat mammary epithelial cells were detected,and comparisons were made between the groups over the same period.Results The incidence and incubation period of DMBA group were lower than the DMBA group and DMBA group,and the differences were statistically significant (P＜0.05).The incidence and incubation period of DMBA group were lower than the DMBA group,and the differences were statistically significant (P＜0.05).There was no significant difference in tumor diameter among the three groups (P＞0.05).The AgNOR count of lactating DMBA group and non lactating DMBA group was lower than the non lactating DMBA group,the difference was statistically significant (P＜0.05).The positive expression rates of C-erb-2,PCNA,Ki-67 and MCM2 in the lactating DMBA group were lower than the non lactating DMBA group and the non lactating DMBA group,and the differences were statistically significant (P＜0.05).Conclusion Fertility and lactation can effectively reduce the probability of dimethylbenzthracene-induced breast cancer in female SD rats.