Abstract: The cyproheptadine (CYP) reversal of multidrug resistance (MDR) and its mechanism in KB_V200 cell line were studies. MTT assay showed that CYP 15.0 μmol·L^-1 could reverse vincristine, adriamycin (ADR) and etoposide resistance in KB_V200 cells by a factor of 5.5, 2.0 and 1.9, respectively. CYP appeared to have no influence on the cytotoxicity of 5-fluorouracil (5-FU) and melphalan (MEL) in the cells. These results indicate that CYP is a MDR reversing agent. CYP 15.0 μmol·L^-1 increased the ADR accumulation in KBV200 cells from 0.68±0.03μg/10⁶ cells to 1.36±0.08 μg/10⁶ cells (P<0.01). CYP 15.0 μmol·L^-1 was shown to obviously increase rhodamine 123 (R123) accumulation in and decrease its efflux from the cells. There was no change in PGP dying intensity under immunocytochemical assay and in mdr1 RNA level through slot blot analysis in the KB_V200 cells exposed continuously to CYP 15.0 μmol·L^-1 for 72 h. These results suggest that CYP acts by inhibiting the pumping function of PGP.