Japanese eel is one of the most valuable species for aquaculture in Taiwan and EastAsia. Artifical reproduction of this spesies attempted for a long time.Several researchgroups succeeded in inducing spawning of the eel by repeated gonadotropin treatment,but can not obtaining eel larval successful.Therefore,in the present study is toinvestigate the osmolarity changes of the Japanese eel in different life stage. To set thesuitable environmental condition for artifical inducing reproduction of the eels.