Our earlier investigation demonstrated that methoxychlor treatment induced hepatic cytochrome P450 2B1/2B2 and 3A proteins in rats and enhanced their respective enzymatic activities. To determine the mechanism of the methoxychlor-mediated induction and whether methoxychlor acts as a frank inducer and not merely by causing stabilization of the cytochrome P450 proteins or of their mRNAs, we assessed the effect of methoxychlor treatment on the transcriptional rate and mRNA levels of these hemeproteins by nuclear run-on and northern blot analyses. Methoxychlor treatment markedly elevated the transcriptional rates of cytochrome P4502B1/2B2 and 3A. Also, higher levels of mRNA of these CYPs were observed. By contrast, mRNA levels of CYP1A1/1A2 were not affected. Using a nuclear run-on method that apparently measures specifically the transcription initiation rate of CYP2B1 and 2B2, we observed that the transcriptional rate of 2B2 was increased substantially more than that of CYP2B1. This differential induction of CYP2B2 versus 2B1 may account for the discrepancy between the levels of induced CYP2B protein and the much lower-than-expected CYP2B-mediated enzymatic activity observed in our earlier study. α§ 1998