Hepatic enzyme induction is often evoked as the cause for a variety of effects in animal studies, e.g. hepatic hypertrophy and secondary thyroid neoplasms in rodents. In clinical practice, enzyme induction often enhances drug clearance and may result in reduced efficacy. For example, carbamazepine or rifampin treatment induces P450 3A in humans, and as a result, dramatically reduces the efficacy of midazolam or cyclosporine. Due to species differences in substrate specificity and the regulation of various drug-metabolizing enzymes, assessing enzyme induction in human tissues is a desirable goal. Since induction often occurs as a result of increased synthesis of mRNA coding for a particular enzyme, induction may be quantified by measuring specific mRNA levels. We describe an approach to quantifying mRNA levels using reverse transcriptase-polymerase chain reaction (RT-PCR). This approach makes use of either radiolabeled PCR primers or fluorimetric quantification of product and does not require the synthesis of a competitor RNA or DNA molecule. Thus, Cyp2B1/2 mRNA can be shown to be induced 17-fold in the H4IIE rat hepatoma cell line. Likewise, Cyp3A and Cyp2A6 mRNAs can be shown to be induced in primary human hepatocytes cultured on collagen-coated plates and treated with rifampin for 72 h. By contrast, mRNA levels for Cyp1A1 and Cyp2E1 were not increased by rifampin treatment. This report demonstrates the potential of this approach for examining a number of mRNAs from drug-treated cultured cells, as a means of assessing metabolic enzyme induction.