Aim To develop a sensitive,rapid and ac-curate LC-MS /MS method for the simultaneous deter-mination of cytochrome P450 probe substrates,inclu-ding caffeine and its metabolite paraxanthine for CYP1 A2,omeprazole and its metabolite 5-hydroxyome-prazole for CYP2C1 9,dextromethorphan and its metab-olite dextrorphan for CYP2D6,midazolam and its me-tabolite 1 ′-hydroxymidazolam for CYP3A4.Methods Probe drugs with the IS diazepam were extracted using ethyl acetate.Gradient elution was performed on an Agilent Eclipse Plus-C1 8 column (50 mm ×2.1 mm, 3.5 μm).The mobile phase consisted of 0.01 % for-mic acid(1 mmol·L -1 ammonium formate)and aceto-nitrile.The flow rate was 0.3 mL·min -1 ,and the in-jection volume was 1 0 μL.The analyte was detected u-sing electrospray ionization(ESI)in positive multiple reaction monitoring(MRM+)mode.The reaction se-lected ions were 1 95.0 /1 38.1 m /z for caffeine, 1 81 .1 /1 24.1 m /z for paraxanthine,346.1 /1 98.1 m /z for omeprazole,362.1 /21 4.1 m /z for 5-hydroxyome-prazole, 272.2 /1 47.1 m /z for dextromethorphan, 258.1 /1 57.1 m /z for dextrorphan,326.1 /291 .1 m /z for midazolam,342.1 /324.1 m /z for 1 ′-hydroxymid-azolam and 285.1 /1 54.0 m /z for diazepam as internal standard.Results The linear ranges of caffeine,pa-raxanthine,omeprazole,5-hydroxyomeprazole,dextro-methorphan, dextrophan, midazolam and 1 ′-hydroxymidazolam were 1 .95 ~2 000,0.98 ~250, 0.48 ~2 000,0.98 ~250,0.98 ~2 000,0.48 ~1 25,1 .95 ~2 000 and 1 .95 ~250 μg·L -1 respec-tively.The RSD of all probe drugs was less than 1 5%and matrix effects in plasma on the ionization of probe drugs were negligible.Conclusion This sensitive and rapid LC-MS /MS method is suitable for determination of the drug/metabolite concentrations in plasma,so as to study the metabolism of CYP1 A2, CYP2C1 9, CYP2D6 and CYP3A4 in depth.