Objective To study the benzo(a)pyrene (B[a]P)-induced mRNA expression of aromatic hydrocarbon receptor (AHR) and cytochrome P4501A1 (CYP1A1) genes in rat liver. Methods Rats were injected intraperitoneally with 5, 10 and 15mg/kg of B[a]P. The total RNAs were extracted from rat livers by RNA purification kit, and the mRNA expression of AHR and CYP1A1 genes was determined by reverse transcription polymerase chain reaction (RT-PCR). β-actin was used as the internal control. The mRNA expression of both AHR and CYP1A1 genes was measured at indicated time points (24, 48 and 72h) after B[a]P treatment at three different concentrations (5, 10 and 15mg/kg). Results The mRNA expression of AHR gene increased in a time-dependent manner at the concentration of 10mg/kg but not at 5 and 15mg/kg of B[a]P. The mRNA expression of CYP1A1 gene differed significantly at 48h and 24h in rat livers treated with 10 and 15mg/kg dosage of B[a]P. The mRNA expression of AHR and CYP1A1 genes increased with B[a]P treatment in a concentration-dependent manner. The time-dependent increase in mRNA expression was shown by AHR but not by CYP1A1 gene with B[a]P (10mg/kg) treatment. Conclusion This study demonstrates that toxic B[a]P increases the mRNA expression of both AHR and CYP1A1 genes in vivo, suggesting that B[a]P may play a role in cancer genesis by this way.