Mifepristone　(RU486)　is　a　chemical　abortifacient　used　by　hundreds　of　millions　of　women　world-wide.　It　has　recently　been　used　in　clinical　trials　for　psychotic　depression　and　cancer　chemotherapy.　Metapristone　is　the　most　predominant　biological　active　metabolite　of　mifepristone,　and　being　developed　as　a　novel　cancer　metastasis　chemopreventive　agent　based　on　its　unique　pharmacological　properties.　In　this　study,　a　novel　rapid　and　sensitive　method　using　UPLC/MS/MS　was　developed　and　validated　for　quantitative　analysis　of　metapristone　in　plasma,　which　used　less　plasma　volume　and　was　demonstrated　to　be　more　simple　and　low-cost　than　the　published　methods.　Metapristone　in　plasma　was　recovered　by　liquid-liquid　extraction　using　1mL　of　ethyl　acetate　and　chromatographic　separation　was　carried　on　a　C18　column　at　35°C,　with　a　gradient　mobile　phase　consisting　of　methanol　and　water　containing　0.1%　(v/v)　formic　acid　at　a　flow　rate　of　0.3mL/min.　The　mass　spectrometric　detection　was　carried　out　using　a　triple-quadrupole　system　via　positive　electrospray　ionization.　Multiple　reaction　monitoring　was　used　for　quantitation　of　m/z　transitions　from　416.3　to　119.9　for　metapristone　and　from　313.1　to　109　for　levonorgestrel　(internal　standard).　Good　linearity　(r2>0.9926)　was　achieved　over　a　concentration　range　from　7.1　to　2840ng/mL　with　a　lower　limit　of　quantification　of　7.1ng/mL　for　metapristone.　The　intra-　and　inter-day　variations　of　the　assay　were　2.4-10.0%　relative　standard　deviation　with　an　accuracy　of　-5.6　to　8.6%　relative　error.　This　newly　developed　method　was　successfully　applied　to　a　pharmacokinetic　study　that　revealed,　for　the　first　time,　that　there　was　a　significant　difference　in　pharmacokinetic　profile　between　genders.　©　2014　Elsevier　B.V.