Abstract: Objective To compare the effi cacies of 5-fl uorouracil(5-Fu) delayed-release implants in local treatment of transplanted breast cancer of Balb/c mice. Methods Fifty Balb/c mice with transplanted breast cancer were randomly divided into 5 groups, 10 in each group. Group A and B were treated with peritumoral implantation of 5-Fu implants, 200 and 100 mg/kg, respectively. Group C was treated with peritumoral injection of 5-Fu solution,200 mg/kg. Group D was treated with intra-abdominal injection of 5-Fu solution, 200 mg/kg. Group E did not receive any treatment. The survival, bodyweight and tumor volume were observed after treatment for 0,3,6,9 and 12 d.After 12 days, all mice were killed, tumors were weighed, tumor inhibition rates were calculated, growth curves were plotted and HER2 expression were detected by immunohistochemistry. Results The growth curves were mild in group A and B. After 12 days, inhibition rates of group A,B,C and D were 50.43%, 41.64%, 27.57% and 22.87%, respectively.There were statistical differences in tumor volume between group A,B and other groups(P<0.05), while there was no statistical difference among group C,D or E. The HER2 expressions in group A,B and C were signifi cantly lower than that in group D(P<0.05). Conclusion Delayed-release 5-fluoroumcil implants can effectively inhibit the growth of breast cancer.