Cytochrome P-450 3A (CYP3A) is a drug-metabolizing enzyme dominant in the human liver. We have designed a useful method for evaluation of induction of CYP3A mRNA by various drugs using HepG2 cells known to retain liver-cellular functions. Using semi-quantitative reverse transcription-PCR (RT-PCR), we demonstrated that cultured HepG2 cells constitutively expressed CYP3A mRNA. This mRNA was expressed at high levels in culture for several days and was further induced by several drugs (e.g. rifampicin (RFP), dexamethasone). Treatment of HepG2 cells with RFP induced CYP3A mRNA in a dose- and time-dependent manner. Cells in culture for 48 h with 1 and 50 micromol/l RFP increased 2.7- and 5.0-fold in CYP3A mRNA expression in comparison with untreated controls, respectively. In contrast, no change in the amount of CYP3A mRNA was observed when the cells were treated with cimetidine which has been shown to inhibit CYP3A activity. Our method using a combination of HepG2 cells and RT-PCR allowed evaluation of the degree of induction of CYP3A mRNA both easily and rapidly.