Sexual behavior and reproductivity of male fIsh were studied as an in vivo screening method of endocrine disruptors. Male medaka (Oryzias latipes) were exposed to 17$eta$-estradiol at nominal concentrations of 2 and 20 $mu extrm{g}$/l for 14 days. After exposure of the chemical, sexual behavior between male medaka and normal female which were injected with prostaglandin $F_{2alpha}$ just before the test, was analysed by using video camera for one hour. Normal control male showed courtship dancing such as following, guarding, dancing and crossing while 17$eta$-estradiol treated male did not show any type oj courtship dancing. Furthermore, fecundity and fertility were significantly decreased in the treated group. It was suggested that analysis of sexual behavior could be a useful endpoint for the screening of the endocrine disruptors.