Angelica sinensis (Oliv.) Diels (DG), Ligusticum chuanxiong Hort. (CX) and Rheum palmatum L. (DH), three well known traditional Chinese medicines (TCM), have been used widely for the treatment of various types of disorders in China. Herb-drug interactions, especially cytochrome P450 (CYP)-mediated interactions, cause an enhancement or attenuation in the efficacy of co-administered drugs. In this study, to assess the possible interactions between TCM and drugs, the effect of water and ethanol extracts of DG, CX and DH on cytochrome P450 were studied in rats. The activities of various CYP enzymes were determined by HPLC method. Treatment of rats with water extracts or ethanol extracts of DG, CX and DH at daily dosages equivalent to 3 g (dry herbal material)/kg all increased the microsome protein contents and decreased the total CYP levels. The water extract of DG strongly increased the activities of CYP2D6 and 3A and the water extract of DH significantly increased the activity of 2D6. The other water extracts all showed inhibition against CYP isoforms. Only the ethanol extract of DG and DH increased the CYP2D6 and 3A activities, respectively, and the other ethanol extracts all decreased the level of CYP isoforms. All extract treatments had significant effects on CYP isoforms activities, whether induction or inhibition, compared with the blank control. Thus, caution should be paid to possible drug interactions of DG, CX, DH and CYP substrates.