A fast and sensitive HPLC-APCI-MS/MS method was developed for the determination of ergosta-4,6,8(14),22-tetraen-3-one (ergone) in rat plasma. The plasma sample containing ergone and ergosterol (internal standard) were simply treated with acetone to precipitate and remove proteins and the isolated supernatants were directly injected into the HPLC-APCI-MS/MS system. Chromatographic separation was performed on a 1.8microm Zorbax SB-C18 column (100mm x 3.0mm) with a 97:3 (v/v) mixed solution of methanol and 0.1% aqueous formic acid being used as mobile phase. Quantification was performed by multiple selected reactions monitoring (MRM) of the transitions with (m/z)(+) 393-268 for ergone and (m/z)(+) 379-69 for the IS. The method was validated in the concentration range of 5-1600ng/mL for ergone. The precision of the assay (RSD%) was less than 10.5% at all concentrations levels within the tested range and adequate accuracy, and the limit of detection was 1.5ng/mL. The absolute recoveries of both ergone and ergosterol from the plasma were more than 95%. The developed method has been successfully applied to the pharmacokinetic study of the drug in SD rats.