Normothermic 3 min lasting perfusion of the isolated rat heart by Krebs--Henseleit solution in which Ca2+ was replaced by EDTA and subsequent perfusion with a Ca2+ containing medium induced structural and metabolic changes demonstrated electron microscopically and histochemically. In contrast to the ischemic reperfusion damage, in calcium paradox, the histochemically studied enzymes alpha-glucan-phosphorylase, lactate dehydrogenase, succinic dehydrogenase, beta-hydroxybutyric dehydrogenase, and ATPases were better preserved in the subendocardial region of the left ventricle.