Abstract: Objective : To investigate the optimal frequency and intensity of Pulsed Magnetic Fields (PMF) in revers-ing multi-drug resistance in breast cancer cell line MCF-7/ADR both in vitro and in vivo. Methods : MCF-7/ADRcells were exposed to PMF of different frequencies (110 Hz and 250 Hz) and intensities (40 mT and 10 mT)for 2 hours. The concentration inhibiting growth by 50% (IC50) was determined by MTT sensitivity assay. Theresistance index was calculated by dividing the IC50 of the resistant cell line MCF-7/ADR by that of theparental cell line MCF-7. The reversal index was calculated by dividing the IC50 of the resistant cell lineMCF-7/ADR after treatment with PMF by that of the resistant cell line MCF-7/ADR. Cells treated with PMFand untreated cells were incubated in rhodamine123 (Rh123) solution for 2 hours at 37?C and the intracellularaccumulation of Rh123 was detected by flow cytometry. Energy-dependent efflux proteins that pump a varietyof chemotherapeutic agents from tumor cells such as P-glycoprotein (p-gp) and multidrug resistance-associat-ed protein (mrp1) on the cell membrane were evaluated by flow immunofluorescence. We used semiquantita-tive PCR (RT-PCR) to detect the expression of MDR1 and MRP1 mRNA and to determine the effect of PMFon the expression of multi-drug resistance genes. An MCF-7/ADR cell xenograft model was established innude mice. The tumor volume and weight were measured to observe the effect of PMF in vivo. Results : PMFeffectively reversed the adriamycin resistance of MCF-7/ADR cells (P<0.01), with a maximum five-fold effectusing 110 Hz and 40 mT. PMF increased the intracellular accumulation of Rh123 by 20% and increased thequantity of p-gp on the cell membrane. PMF had little effect on mrp1 mRNA but decreased the expression ofMDR1 and MRP1 proteins. PMF combined with ADR inhibited tumor proliferation in our xenograft model (P<0.01). Conclusion : PMF decreases MDR1 and MRP1 gene expression in the MCF-7/ADR breast cancer cellline both in vitro and vivo.