Abstract: A sensitive LC-MS/MS method to determine cocaine and its major metabolite benzoylecgonine in guinea pig's hair has been established. About 20 mg of decontaminated hair sample was hydrolyzed with 0.1 mol·L^-1 HCl at 50 ℃ overnight, in the presence of cocaine-D₃ and benzoylecgonine-D₈ used as internal standards, and then extracted with dichlormethane. The analysis was performed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Positive electrospray ionization (ESI+) and multiple reactions monitoring (MRM) mode were used. The limit of detection (LOD) for cocaine and benzoylecgonine was 1 pg·mg^-1. The calibration curves of extracted standards were linear over the range from 5 pg·mg^-1 to 250 pg·mg^-1 (r²≥0.999 7). The method was validated and applied to the analysis of guinea pig's hair after a single dose administration of cocaine hydrochloride. Cocaine and benzoylecgonine were not only detected, but also quantified in guinea pigs hair.