The changes of triiodothyronine ( T3 ) and thyroxine ( T4 ) levels in serum , eggs and larvae were measured by radioimmunoassay (RIA) after T4 injection in female Japanese eel spawners at doses of 5 , 10 , 15 and 20 mg ?(500g) 1 body weight fish. As results , T3 level reached up to peak value following T4 level up to peak value in maternal serum after 24 h of exogenous T4 injection. Apparently , there was conversion of T4 into T3 in the broodstocks which suggests the presence of the enzyme , 5’ monodeiodinase , in Japanese eel. Meanwhile , along with the T3 and T4 levels in serum up to peak value , the T3 and T4 level in eggs also reached up to peak value. It further indicated that T3 and T4 in maternal circulation could be transferred into the oocytes and subsequently into the larvae. After readjusting the times and doses of T4 injection , it was discovered that the most suitable doses of exogenous T4 injection should be 10 -15 mg?(500 g) 1 body weight and the optimal opportunity of T4 injection should be 12 h before spawning under the condition of 21 ℃ water temperature. Compared with the control , the rate of larval survival in each stage of development tended to show remarkable increase ( P < 0. 01) after spawners were treated with T4. The rate of 92day2old larval survivalwaselevatedfrom1.3%ofcontrolgroupto22.0%oftreatedgroup. However,thelarvaecouldnotsurviveultimately. It suggests that thyroid hormone may not be the only factor of affecting the larvae survival.