Although 1,1-dichloro-2,2-bis(p-chlorophenyl)-ethylene (p,p-DDE),the major and most persistent metabolite of dichlorodiphenyl-trichloroethane (DDT),was continually detected in wild fishes that showed abnormal gonad development such as intersex,little is known about the impact of p,p-DDE exposure on gonad development in fishes.To survey the effects of p,p-DDE on gonadal development and gene expressions,male juvenile (20-d post hatch) Japanese medaka (Oryzias latipes) was exposed to 1,5,20,and 100 μg/L p,p-DDE for two months.Increased hepatosomatic index (HSI) and decreased gonadosomatic index (GSI) were found in the p,p-DDE-treated groups.Intersex was found in 100 μg/L p,p-DDE exposure group,as well as 100 ng/L 17α-ethynylestradiol (EE2)group.By quantitative real-time RT-PCR,it was found that gene expressions of vitellogenins (VTG-1,VTG-2),choriogenins (CHG-H,CHG-L),and estrogen receptor α (ER-α) in the liver of the fish were significantly up-regulated by p,p-DDE exposure.VTC-1 and VTG-2 were recommended as the preferred biomarker for assessing anti-androgenic p,p-DDE because they were the highest up-regulated among the genes and showed good dose-response relationship.The up-regulated ER-α suggested that a potential synergetic effect would occur when p,p-DDE coexists with other ER-α-binding endocrine-disrupting chemicals (EDCs).