Catalpol is a Chinese herb ingredient with potential for the treatment of neurodegenerative disorders. A high-performance liquid chromatography-atmospheric pressure chemical ionization-tandem mass spectrometry (HPLC-APCI-MS/MS) method was developed and validated to investigate the pharmacokinetics and biodistribution of catalpol in both the plasma and cerebrospinal fluid (CSF) of rats. Methanol was used to precipitate the protein from the biosamples, and the supernatant was collected for the assay. A Diamonsil C18 column (150 mm × 4.6 mm, 5 μm) with a mobile phase of methanol-ammonium acetate (20 mM) (50:50, v/v), a 0.6 mL/min flow rate and a total run time of 3 min was used to separate catalpol and aucubin (internal standard, IS). A triple-quadrupole tandem mass spectrometer with an atmospheric pressure chemical ionization (APCI) source in the positive ion mode was operated in the multiple reaction-monitoring (MRM) mode to determine the concentration of catalpol at m/z 380.0-165.0 and that of IS at m/z 364.0-148.9. The linear range was 10-50000 ng/mL for plasma samples and 20-5000 ng/mL for CSF samples. At both the lower limit of quantification (LLOQ) and three levels QC (quality control) concentrations, the RSD was less than 12.9%, and the bias was -10.0% to 7.1%. The extraction recoveries for catalpol ranged from 72.9% to 109.5% from both rat plasma and CSF. The catalpol was administered to rats in 6 mg/kg doses via intravenous (i.v.) injection, and the pharmacokinetics and biodistribution studies were performed in both plasma and CSF. The results demonstrated that catalpol could be transported into the CSF via the AUC(CSF)/AUC(plasma) of 5.8% with a half-life (t(1/2)) of 1.5 h. Overall, the established HPLC-APCI-MS/MS method is rapid and sensitive, and has been successfully used to quantify catalpol in both plasma and CSF.