Abstract: OBJECTIVE To establish an LC-MS/MS method for the determination of simvastatin in cynomolgus monkey plasma,and compare its stability differences in three species of human, cynomolgus monkey and rat plasma.METHODS Validated LC-MS/MS method was established and used for analysis of simvastatin samples which were prepared in the plasma of three species and stored under different conditions.RESULTS The linearity of calibration curve was 0.5-50 μg·L-1 and the limit of quantitation was 0.5 μg·L-1. The inter- and inner- day precision were below 6.67% and the accuracy of the method was 99-109%. Drug concentration of cynomolgus monkey plasma samples being placed 24 h at room temperature decreased 70%，drug concentration of SD rat plasma samples being placed 4 h and 24 h at room temperature decreased 75% and 80%,drug concentration of human plasma samples being placed 24 h at room temperature decreased 30%.Samples frozen at -80 ℃ were mostly stable, except for that of rat plasma samples.CONCLUSION The established method is suitable for the determination of simvastatin in cynomolgus monkey plasma. Stability of simvastatin varies greatly in different plasma and shows species-related differences.