Aim To study the effects of triiodothyronine on the Bax and Bcl-2 of immature cadiocytes while given ischemical reperfusion injury. Methods The cadiocytes of neonate rat were culrured in vitro,and the ischemical reperfusion injury of cadiocytes were induced by anoxia-reoxygenation.Triiodothyronine(T3) was administered before anoxia-reoxygenation at a concentration of 1 μg/L,Bcl-2 mRNA and Bax mRNA were detected by polymerase chain reaction(PCR). Results The Bcl-2 were down-regulated and Bax were up-regulated while given T3 shortly before anoxia-reoxygenation.At the same time,the apoptosis of cadiocytes were reduced. Conclusions Giving triiodothyronine before anoxia-reoxygenation would enhance the ablity of immature cadiocytes against ischemical reperfusion injury,and could reduce the apoptosis of cadiocytes.This effect was associated with the expression of Bcl-2 and Bax.