Objective: To observe the effect of Sophorae Tonkinensis Radix et Rhizoma (STR) on the activity of cytochrome P450 (CYP450) in rats. Method: Wistar rats were randomly divided into normal control group and STR low dose (1 g·kg-1) and high dose (12 g·kg-1) groups. STR 1

12 g·kg-1 groups were treated with STR decoction by intragastrical administration

while the normal control group was given distilledwater of the same volume once a day for 21 consecutive days. The blood was collected after 21 day treatment. The levels of serum alanine aminotransferase (ALT)

aspartate aminotransferase (AST)

alkaline phosphatase (ALP) and total bilirubin (TB) were measured by using automatic biochemical analyzer. The liver weight index was calculated and the pathological changes of hepatic tissues were observed. The activity of the subtype enzymes of CYP450 including CYP1A2

CYP2E1

CYP3A1

CYP2C9 and CYP2D6 of the liver was measured by probe drugs. Result: Compared with the normal control group

the STR high dose group demonstrated certain liver toxicity

including the increased activity of ALT

ALP and liver weight index (P<0.01)

spotty necrosis and inflammatory cell infiltration in the liver of some animals. The high dose of STR induced the activity of CYP3A4

CYP2C9 and CYP2D6 (P<0.05 or P<0.01). Compared with low dose of STR

high dose of STR induced more activity of CYP3A4 (P<0.01). Conclusion: High dose of STR can induce the activity of CYP3A4

CYP2C9 and CYP2D6

and it is more sensitive to CYP3A4

suggesting that it should pay more attention to the drug interactions when STR is used in the combination with other drugs which also have the effects on CYP3A4.