A rapid, accurate, and sensitive liquid chromatography/mass spectrometry (MS)/MS method for the quantitative determination of valproic acid (VPA) and its metabolite, 2-propyl-4-pentenoic acid (4-ene VPA), in monkey plasma was developed and validated. The sample extraction was performed using hydrophilic-lipophilic balance cartridge. The analytes were separated on a Kinetex C18 ($2.1mm{ imes}100mm$, 2.6 ${mu}m$) analytical column under a mobile phase consisting of 10 mM ammonium formate (pH 8.0)/methanol (20/80, v/v) and isocratic flow at 0.15 mL/min. The tandem mass spectrometer was operated in negative electrospray ionization with selected ion monitoring conditions, 143.0, 141.0, and 121.0 for VPA, 4-ene VPA, and benzoic acid (internal standard), respectively. The linearity of calibration curve ranging from 0.1 to 20 ${mu}g/mL$ was at least 0.9996 (coefficient of correlation, r) for both analytes. Intra- and inter-day precisions for both analytes were lower than 15%, resulting from quality control (QC) samples at concentration of 0.2 (low QC), 1.6 (middle QC), and 16 (high QC) ${mu}g/mL$ except at the lower limit of quantification (LLOQ) (0.1 ${mu}g/mL$) level, which was less than 20%. The intraand inter-day accuracies were within ${pm}15%$. The recoveries were 84.4-90.8% for VPA and 88.2-100.6% for 4-ene VPA. Both analytes were stable throughout short-term temperature, post preparation for 24 h, and three freeze/thaw cycles, validating that this method could be applied to toxicokinetic and pharmacokinetic studies.