The hexane and ether extracts from the roots of Angelica dahurica caused a significant inhibition of hepatic drug-metabolizing enzyme (DME) activity. Through systematic fractionation by $SiO_2;column$ and vacuum liquid chromatography monitoring by bioassays, three furanocoumarins, phellopterin, byakangelicin and tert-O-methylbyakangelicin were isolated as active principles. These components have biphasic responses, both inhibitory and inducing effects on DME system. Tert-O-methyl byakangelicin was found to have the strongest enzyme inhibitory potency.