A reduction of the biological half life of toluene in blood in the simultaneous exposure to toluene and alcohol vapors has been reported. To clarify the cause of this reduction, gene expression of CYP mRNA in liver of rats exposed to bi-component organic vapors was investigated. Wistar male rats were repeatedly exposed to 500 ppm of toluene and 300 ppm of 1-butanol vapors individually and simultaneously by inhalation 6 hours a day, five days a week for 4 weeks. After the exposure, the rats were sacrificed and the livers were collected and homogenized. RNA was extracted from the livers, and gene expression of CYP mRNA was observed by reverse transcription-polymerase chain reaction (RT-PCR). The gene expression of CYP3A2 in the simultaneous exposure group was significantly higher than that in the toluene exposure group. However, there was no significant difference in that of CYP1A2, CYP2B1, CYP2C11, CYP2E1 and CYP4A1.