Levo-tetrahydropalmatine (l-THP), one of the main active alkaloids isolated from Rhizoma corydalis, was recently found to elicit profound effects on the dopaminergic system in the striatum, which plays an important role in regulating nociception. A rapid and sensitive method based on microdialysis combined with liquid chromatography-tandem mass spectrometry was developed for the determination of l-THP in the rat striatum. Microdialysis probes were stereotactically placed in the striatal hemisphere, and l-THP was measured from the microdialysates collected using LC-MS/MS. Reverse-phase LC separation was accomplished on a Diamonsil™ C18 column (50mm×2.1mm ID, 5μm) with the mobile phase composed of methanol-water (50:50, v/v) at a flow rate of 0.2ml/min. The method had a chromatographic total run time of 5min. Detection was performed in electrospray positive mode and quantification was executed in selected reaction monitoring mode. The following transitions were monitored: m/z 356.0→191.9 for l-THP and 256.0→167.1 for the internal standard diphenhydramine. The method was sensitive with a lower limit of quantitation (LLOQ) of 0.1ng/ml for l-THP, with good linearity in the range of 0.1-1000ng/ml (r(2)≥0.999). All the validation data, such as accuracy, precision, and inter-day repeatability were within the required limits. The method was successfully applied to pharmacokinetic study of the l-THP in the rat striatum.