Abstract: OBJECTIVE To evaluate the in vitro inhibitory effects of five kinds of herbal constituents (honokiol, magnolol, geniposide, chlorogenic acid, astragaloside Ⅳ) on CYP1A2, CYP3A and CYP2D in human and rat liver microsomes. METHODS The activities of CYP1A2, CYP3A and CYP2D in human and rat liver microsomes were evaluated by detecting turnovers of its substrates after treatment with the five kinds of herbal constituents in vitro by HPLC. Phenacetin, midazolam and dextromethorphan were used as substrates of CYP1A2, CYP3A and CYP2D, then the inhibition ratio and IC50 were calculated. RESULTS The IC₅₀ of honokiol on CYP1A2 in human and rat were 5.5, 3.9 μmol·L^-1 and CYP2D in human and rat were 35.3 and 46.7 μmol·L^-1, respectively. The IC₅₀ of magnolol on CYP1A2 in human, CYP1A2 and CYP2D in rat were 23.8, 29.1 and 39.9 μmol·L^-1, respectively. The IC₅₀ of geniposide, chlorogenic acid and astragaloside Ⅳ on the three kinds of CYP enzyme subtypes were greater than 100 μmol·L^-1. The IC₅₀ of honokiol on human and rat CYP3A were >100 μmol·L^-1. The IC₅₀ of magnolol on human CYP3A, CYP2D and rat CYP3A were also >100 μmol·L^-1. CONCLUSION The activities of CYP1A2 and CYP2D in human and rat liver microsomes are respectively inhibited by honokiol, the activities of CYP1A2 in human and rat liver microsomes and CYP2D in rat liver microsomes are respectively inhibited by magnolol, which are all in a dose-dependent manner in vitro.