Abstract: Objective: To investigate the inhibitary effect of different concentrations of arsenic trioxide(As₂O₃) on breast cancer cells MDA-MB-231 and on the activity of telomerase hTERT-mRNA so as to provide theoretical and experimental basis for the treatment of breast cancer.Methods: MDA-MB-231 cells were cultured with As₂O₃ diflerint cokcenfrations.The growth inhibition was analyzed by MTT assay;the telomerase activity was determined by TRAP-PAGE-SILVER staining;the hTERT-mRNA expression was examined by RT-PCR assay.Results: As₂O₃ could obviously inhibit the growth of MDA-MB-231 cells,which was related with the concentrations of As₂O₃ and time of culture.Its 24-hour inhibitory concentration(IC₅₀) was 24.73 μmol/L and 48-hour inhibitory concentration(IC₅₀) was 6.99 μmol/L.Treated with As₂O₃(10-40 μmol/L) for 24-48 h,the telomerase activity and hTERT-mRNA expression of MDA-MB-231 cells were greatly inhibited.Conclusions: As₂O₃ can inhibit the growth of MDA-MB-231 cells significantly.Treated with As₂O₃,the activity of telomerase and telomerase hTERT-mRNA can be inhibited remarkably,and the effects are dependent on the concentrations and time of culture.