Abstract: Previous publications showed that the value of LLOQ (lowest limit of quantification) for doxazosin and its enantiomers in biological samples were above 0.1 ng·mL^-1. The present study was designed to establish a new liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the quantification at very low concentration of (-)doxazosin in rat plasma after intravenous administration of (-)doxazosin (3.0 mg·kg^-1). The plasma samples containing prazosin as an internal standard were extracted by solid-phase extraction (SPE) and separated on Acquity BEH C₁₈(50 mm×2.1 mm, 1.7 μm) column under alkaline conditions of the mobile phase. (-)Doxazosin was monitored under positive ionization condition by multiple reaction monitoring (MRM) with an ESI source. The good linear range of (-)doxazosin varied from 10.4 pg·mL^-1 to 13 ng·mL^-1(r=0.9922), and the lowest limit of quantification was 10.4 pg·mL^-1. An assessment of the matrix effect using post-extraction spiking method and post-column infusion method demonstrated that co-eluting matrix components did not significantly influenced the ionization of (-)doxazosin and prazosin (IS). Using the new method, we accurately measured (-)doxazosin concentration at 48 h after intravenous administration in the rats, and the concentration was 0.0344±0.0102 ng·mL^-1. The method is specific, sensitive, and suitable for determining (-)doxazosin at very low concentration in rat plasma samples.