It has been demonstrated that nonylphenol (NP) exerts estrogenic activity. Quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR) was used to study the VTG-I , VTG-II , CHG-H and CHG-L genes expression in the liver of juvenile medaka exposed to NP at 1, 10, 50, 100 microg/I. for 60 days. The results show that the VTG-I , VTG-II, CHG-H and CHG-L genes expression in the liver of juvenile medaka are induced even at 1 microg/L, significantly. It should be noted that the lowest-observed-effect concentration (LOECs) based on the hepatic vitellogenin (VTG) induction is about 1 microg/L, suggesting that quantitative real-time RT-PCR can detect the estrogenic activity of NP at relatively low concentration, and there is a potential application in evaluating the estrogenic activity of NP in aquatic environment.