Objective: The aim of this study was to establish a rapid and accurate method in order to evaluate the activity of CYP3A4 by the high-performance liquid chromatography (HPLC). The activity of CYP3A4 was evaluated to explore the protective effect and mechanism of Guipi Tang on rats' acute liver injury which is resulted from the extract from Tripterygium wilfordii by Ethanol. Method: Totally 50 rats were randomly divided into control group

model group

Guipi Tang group

P450 induction group

and P450 inhibition group. The control group and the model group was perfused with physiological saline

the Guipi Tang group was perfused with Guipi Tang (9.0 g · kg-1). The P450 induction group and P450 inhibition group have received injection of DXM(100 mg ·kg-1) and KCZ(80 mg ·kg-1) in intraperitoneal respectively. After ig 4 days

and perfused with T.wilfordii (3.25 g · kg-1) for 3 days

the model was established. The activity of CYP3A4 of rat liver microsome was measured by HPLC with midazolam as a probe drug. Result: The retention time of midazolam appeared at 8.960 min under the condition of HPLC

while there were no interference peaks. Its suitable linear range was 0.25～20 μmol · L-1 (r=0.999 9)

which met the requirement of the whole examination. Compared with the control group

the activity of CYP3A4 in model group decreased(P<0.05). Compared with the model group

Guipi Tang and P450 induction group increase the CYP3A4 activity(P<0.05). Conclusion: The evaluation method of CYP3A4 activity is accurate. T. wilfordii can inhibit CYP3A4 activity significantly. Guipi Tang could reduce the toxicity of T. wilfordii and protect liver through inducing the CYP3A4 activity.