Aim To develop a liquid chromatography electrospray-ionization tandem mass spectrometry (LC- MS/MS) method for simultaneous determination of bentysrepinine (Y101) and its metabolites M8 and M9 in rat plasma and to investigate the effect of verapamil, an inhibitor of P-glycoprotein (P-gp) , on the pharma¬cokinetics of Y101, a substrate of P-gp, in rats. Methods SD rats were divided randomly into two groups; ( 1) Y101 only as a control group, received an oral dose of 60 mg • kg"1 Y101; (2) Verapamil plus Y101 as an experimental group, received an oral dose of 60 mg • kg"1 Y101 in combination of 25 mg • kg"1 verapamil. The plasma concentrations of Y101 and its metabolites were determined by LC-MS/MS method af¬ter intragastric administration, and the pharmacokinetic parameters were calculated using non-compartmental a- nalysis. Results We successfully developed and fully validated a LC-MS/MS method, which simultaneously determined the concentration of Y101 and its metabo¬lites in rat plasma. The AUC0_t for Y101 and M9 in experimental group increased to 1.71-fold and 1.58- fold in comparison of control group. At the same time, the plasma clearance of Y101 and M9 decreased to 60% of control. However, we did not find any differ¬ence in AUC0_l and plasma clearance for M8 between two groups. Conclusions The validated LC-MS/MS method is sensitive and rapid for the determination of Y101 and its metabolites in rat plasma and was suc¬cessfully applied to the pharmacokinetic study in rats. Verapamil, a P-gp inhibitor, significantly increases the exposure of Y101 and its metabolites in vivo, indicating the adjustment of Y101 dosage for combined adminis¬tration is needed in clinical practice.