The changes in adenosine triphosphate (ATP), creatine phosphate (CP) and lactic acid (LA) contents of guinea pig hearts were studied during the cardioplegia and recovery phase. 1) ATP and CP contents in 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 concentration of Ca2+ in the cardioplegic solutions tested, while the recovery of ATP contents was faster with 15 mM K+, 0.1 mM Ca2+ cardioplegic solutions. 2) LA contents were increased during the cardioplegia and decreased with the reperfusion of normal Tyrode solution. 3) The more recovery time (up to 3 hrs), the more CP contents were recovered with the reperfusion of normal Tyrode solution faster than those of ATP. And LA contents were decreased as the duration of recovery time. These results suggest that Ca2+ and K+ concentration in the cardioplegic solution is one of the major factors influencing the recovery of cardiac tissue from the cardioplegia.