5-methoxycanthin-6-one, a major canthinone alkaloid isolated from Picrasma quassioides, exhibited significant pharmacological activities. In this study, a rapid and sensitive LC-MS/MS method was established and validated for the determination of 5-hydroxy-4-methoxycanthin-6-one in rat plasma. Small quantities (20 muL) of plasma sample were used for sample preparation. 5-Hydroxy-4-methoxycanthin-6-one and an internal standard (IS, caffeine) were separated using an ACQUITY HSS T3 column (50 x 2.1 mm, 1.7 mum; Waters, Milford, MA, USA). The mobile phase was composed of 0.1% formic acid in water and acetonitrile. Precursor-to-product ion transitions were m/z 267.0 --> 168.2 and m/z 195.0 --> 138.1 for quantitative monitoring of 5-hydroxy-4-methoxycanthin-6-one and IS, respectively. The assay was linear over the concentration range of 0.5-500 ng/mL (r > 0.99) with the lower limit of quantification 0.5 ng/mL. Other parameters, including intra- and inter-day precision and accuracy, carryover, stability, extraction recovery, matrix effect, and dilution effect, were within acceptable limits. The validated method was successfully applied to pharmacokinetic study in rats after intravenous (5 mg/kg) and oral (10, 25, 50 mg/kg) administration of 5-hydroxy-4-methoxycanthin-6-one. The result indicated that 5-hydroxy-4-methoxycanthin-6-one was quickly absorbed into the blood and reached the highest concentration at ~33.0-42.0 min, with moderate elimination half-life (0.85-2.11 h) and low bioavailability (16.62-24.42%) after oral administration. The study provided valuable information that can be used as a reference for studying other canthinone alkaloids. CI - (c) 2020 John Wiley & Sons, Ltd.