Drug interactions which affect drug metabolism are of clinical importance. It is, however, difficult to estimate drug interactions in human from results obtained in animal experiments. In our previous study, we demonstrated that a combination of the HepG2 cell line and semiquantitative reverse transcription-PCR (RT-PCR) could be used to evaluate the degree of CYP3A mRNA induction by various drugs. Using an RT-competitive PCR (RT-cPCR) with beta-actin as the standard in this study, the constitutive and rifampicin (RFP)-induced expression of CYP3A4, CYP2C9, CYP2E1, and CYPA2 mRNA in the HepG2 cells could be quantitatively and reproducibly determined. 120 h-treatment of HepG2 cells with 50 mu mol/1 RFP induced maximally 8.4- and 6.0-fold the expression of CYP3A4 and CYP2C9 mRNA respectively in comparison with untreated cells. On the other hand, mRNA level in CYP2E1 and CYP1A2 was not significantly changed by 50 mu mol/l RFP after 24 to 120 h, To our knowledge, we report for the first time quantitative profiles of CYPs mRNA in HepG2 cells. This study demonstrates the efficiency of a combination of HepG2 cells and RT-cPCR in the evaluation of CYPs mRNA-induction by drugs.