Ischemia and reperfusion of rat hearts were produced by Langendorff per-fusion technique. The energy tranduction of the mitochondria isolated from the rat heartswas studied. The initial rate of proton ejection and eldectron transfering were measured.There was no significant difference of H~+/2_e ratio between the ischemic group and thecontrol group which was reduced only in the reperfusion group. The longer was the is-chemic period before reperfuaion, the smaller would be the H~+/2_e ratio. The lowering ofH~+/2_e ratio occurred earlier than that of the ADP/O ratio. RCR and ADP/O in shortperiod ischcmia group and reperfusion group were both much higher than that of the con-trol group.