This study used for the first time LC-MS/MS for the analysis of mitragynine (MIT), a mu-opioid agonist with antinociceptive and antitussive properties, in rat plasma. Mitragynine and the internal standard (amitriptyline) were extracted from plasma with hexane-isoamyl alcohol and resolved on a Lichrospher RP-SelectB column (9.80 and 12.90 min, respectively). The quantification limit was 0.2 ng/mL within a linear range of 0.2-1000 ng/mL. The method was applied to quantify mitragynine in plasma samples of rats (n=8 per sampling time) treated with a single oral dose of 20 mg/kg. The following pharmacokinetic parameters were obtained (mean): maximum plasma concentration: 424 ng/mL; time to reach maximum plasma concentration: 1.26 h; elimination half-life: 3.85 h, apparent total clearance: 6.35 L/h/kg, and apparent volume of distribution: 37.90 L/kg.