Quantitative reverse-transcriptase polymerase chain reaction was used to determine the content of mRNA derived from four CYP3A genes (CYP3A2, CYP3A9, CYP3A18, and CYP3A23) in rat liver. CYP3A2 and CYP3A9 gene expression was age- and sex-dependent, whereas CYP3A18 and CYP3A23 mRNA were observed before and after puberty at fairly constant levels that were about 20% higher in males than in females. CYP3A9 mRNA was detected only in adult rats, with a nearly twofold higher expression in females. CYP3A9 induction was different from other CYP3A isoenzymes, since phenobarbital was a more effective inducer than dexamethasone and clotrimazole. The results presented for CYP3A23 are those anticipated for CYP3A1, which may be an allelic variant of CYP3A23 not detected in these experiments. These data show that rat CYP3A genes are variably expressed depending on age, sex, or inducer type.