The effects of Pb, Mo, As and Rb on spermatogenesis were investigated by using testicular organ culture of Japanese eel (Anguilla japonica). Treatment with these trace elements together with 11-ketotestosterone (KT) significantly inhibited KT-induced germ cells proliferation, suggesting that Mo, Rb, Pb and As directly interrupt KT-induced spermatogenesis. We also investigated the effects of As on in vitro KT synthesis in testis. KT production induced by human chorionic gonadotropin (hCG) was inhibited by low doses of As. This result suggested that As also may directly inhibit KT production.