AIM: To investigate the effect of tamoxifen (TAM) on multidrug resistance (MDR) of colorectal carcinoma in vivo and its relationship with estrogen receptor (ER). METHODS: Multidrug resistance was determined by means of semi-quantitative retro-transcription polymerase chain reaction (RT-PCR) to test mdr1 gene mRNA and ER expression was studied by immunohistochemistry. Tumor tissues from three cases of human colon carcinoma, which had mdr1(+)/ER(+),mdr1(+)/ER(-), mdr1(-) expressions, were planted subcutaneously in the neck of nude mice to establish three xenograft models. These models were subdivided into four subgroups randomly: Doxorubicin (DOX)-treated group, TAM-treated group, DOX and TAM group and control group. The dimensions of these xenografts were measured after each course of treatment and the xenografts were removed at the end of the experiments for measurements of weight and the variation of mdr1 mRNA level with RT-PCR. In each course, TAM [15 mg/(kg/d)] was administrated orally per day in the first seven days and DOX (3.6 mg/kg) was injected peritoneally on the first day. Data was evaluated by q and t tests. RESULTS: In the animal models with mdr1(-) tumor, the weights and volumes of the planted tumor in DOX group [(39.1±2.29) mg, (31.44±1.61) mm3] and TAM and DOX group [(38.72±2.56) mg, (31.31v1.74) mm3], which were lesser than that of control group [(45.48±3.92) mg, (36.42±2.77) mm3, P= 0.037, P= 0.016 respectively] significantly. In the animal models with mdr1(+)/ER(+) tumor, the weights and volumes of planted tumor were not affected by DOX or TAM treatment; however, in TAM and DOX group [(425.5±28.58) mg, (340.35±22.28) mm3], they were significantly less than that of control group [(634.23±119.41) mg, (507.45±93.34) mm3, P= 0.022, P = 0.045 respectively], which are similar to that in the models with mdr1(+)/ER(-) tumor. No significant changes were found in the expressive level of mdr1 mRNA following these treatments. CONCLUSION: The expression of mdr1 gene corresponds to the sensitivity of colon cancer to anti-tumor drugs in vivo. TAM can reverse the MDR of colorectal carcinoma in nude mice, which is independent of the expression of ER; however, no change was observed in the expressive level of mdr1 mRNA. AIM: To investigate the effect of tamoxifen (TAM) on multidrug resistance (MDR) of colorectal carcinoma in vivo and its relationship with estrogen receptor (ER). METHODS: Multidrug resistance was determined by means of semi- quantitative retro-transcription polymerase chain reaction (RT-PCR) to test mdr1 gene mRNA and ER expression was studied by immunohistochemistry. Which Tumor tissues from human breast carcinoma, which had mdr1 (+) / ER (+), mdr1 (+) / ER These models were subdivided into four subgroups randomly: Doxorubicin (DOX) -treated group, TAM-treated group, DOX and TAM group and control group . The dimensions of these xenografts were measured after each course of treatment and the xenografts were removed at the end of the experiments for measurements of weight and the variation of mdr1 mRNA level with RT-PCR. In each course, TAM [15 mg / ( kg / d)] was administrated ora lly per day in the first seven days and DOX (3.6 mg / kg) was injected peritoneally on the first day. Data was evaluated by q and t tests. RESULTS: In the animal models with mdr1 (-) tumor, the weights and volumes of the planted tumor in DOX group [(39.1 ± 2.29) mg, (31.44 ± 1.61) mm3] and TAM and DOX group [(38.72 ± 2.56) mg, (31.31v1.74) mm3], which were lesser than that of In the animal models with mdr1 (+) / ER (+) tumor, the weights and volumes of planted (45.48 ± 3.92) mg, (36.42 ± 2.77) mm3, P = 0.037, however, in TAM and DOX group [(425.5 ± 28.58) mg, (340.35 ± 22.28) mm3], they were significantly less than that of control group [(634.23 ± 119.41) mg, (507.45 ± 93.34) mm3, P = 0.022, P = 0.045 respectively], which are similar to that in the models with mdr1 (+) / ER (-) tumor. No significant changes were found in the expressive level of mdr1 mRNA following these treatments. CONCLUSION: The expressio n of mdr1gene corresponds to the sensitivity of colon cancer to anti-tumor drugs in vivo. TAM can reverse the MDR of colorectal carcinoma in nude mice, which is independent of the expression of ER; however, no change was observed in the expressive level of mdr1 mRNA .