Intracellular Ca2+-overload in the myocardium can be induced not only after readmission of Ca2+-containing fluid to rat hearts previously perfused with a Ca2+-free buffer, a phenomenon called "the calcium paradox", but also during administration of a Ca2+-ionophore to cardiac tissue. In rat hearts, the myocardial damage induced by the Ca2+ paradox was more pronounced than that after administration of the Ca2+-ionophore A23187, as indicated by the amount of lactate dehydrogenase released.