To observe the effects of water extraction of processed Polygonum multiflorum on enzymatic activities and mRNA expressions of CYP1A2 in rats. Methods:The water extraction of processed Polygonum multiflorum was obtained through extraction with water and vacuum drying and the main components were detected by HPLC. Rats were randomly divided into the blank control group and 2 g/kg BW and 20 g/kg BW the water extraction of processed Polygonum multiflorum group. The administration group was continuously given with water extraction of processed Polygonum multiflorum 10 mL/kg a day, and the blank control group was given the same volume distilled water. After successive administration of 7 d, the liver microsomes were prepared from isolated rat liver, and metabolism elimination rates of phenacetin of CYP1A2 enzyme substrate were assayed by HPLC to determine CYP1A2 enzyme activities in rat liver microsomes of each group. Meanwhile, total RNA of rat liver were extracted to determine the mRNA expressions of CYP1A2 in rat liver by qRT-PCR. Results:Compared with those in blank control group, the enzymatic activities and the mRNA expressions of CYP1A2 in 20 g/kg water extraction of processed Polygonum multiflorum treated group were significantly decreased(enzymatic activities of CYP1A2, P<0.01; mRNA expressions of CYP1A2, P<0.05). Conclusion:20 g/kg water extraction of processed Polygonum multiflorum has significant inhibitory effect on the enzymatic activities and the mRNA expressions of CYP1A2 in rat’s liver microsomes.