The changes in membrane potential and intracellular Ca²⁺ activities along with adenosine triphosphate (ATP) and creatine phosphate (CP) contents of guinea pig hearts were studied during the cardioplegia and recovery phase. The results obtained were summarized as follows: 1) A low concentration of Ca²⁺ in the cardioplegic solution caused significant changes in membrane potential during the cardioplegia and action potential configurations of the first beat after cardioplegia. 2) Intracellular Ca²⁺ activities were decreased significantly during cardioplegia and increased when the cardiac tissue was reperfused with the normal Tyrode solution. The magnitude of increase in intracellular Ca²⁺ activities was larger in the 0.1 mM Ca²⁺-cardioplegic solution than the 1.8 mM Ca²⁺-solution. 3) ATP and CP contents of cardiac ventricular tissue were decreased during the cardioplegia, regardless of Ca²⁺ concentration in the cardioplegic solutions, and CP contents were recovered with the reperfusion of normal Tyrode solution faster than those of ATP. And, there were no significant differences in the recovery of CP contents with different concentrations of Ca²⁺ in the cardioplegic solutions tested, while the recovery of ATP contents was faster with 15m M K⁺, 0.1mM Ca²⁺ cardioplegic solution. These results suggest that the Ca²⁺ concentration in the cardioplegic solution is one of major factors influencing the recovery of cardiac tissue from the cardioplegia.