Abstract: The effects of ADSD and MT via implantation on testosterone (T)and 17β -estradiol (17β -E2)levels in the serum of Japanese eel , Anguilla japonica were examined .In the female eel , serum T and 17β -E2 levels were significantly increased after implantation of ADSD , and then decreased gradually .With increasing times of implantation of ADSD , serum T levels on the 15th day after implantation decreased , whereas serum 17β -E2 levels rose .Implantation of MT decreased the serum T levels and increased serum 17β -E2 levels of female eels as compared to the controls .In the male eel , serum T and 17β-E2 levels on the 15th day after implantation of ADSD rose gradually with increasing times of implantation .Serum T levels on the 15th day after implantation of MT also rose with increasing times of implantation , however , significantly higher serum 17β -E2 level was observed only on the 15th day after third implantation of MT .The result suggested that the effects of ADSD or MT via implantation on T and 17β -E2 level in the serum of Japanese eel were sex dependent .