The effect of 7-oxo PGI2 on function and metabolism of postischemic reperfused (30-min ischemia and 30-min reperfusion) rat hearts was studied with special regard to calcium overload as one of the main factors of the postischemic reperfusion damage to the heart. The drug (50 micrograms/kg i.p.) was applied 48 h prior to starting the experiments on isolated rat hearts (Langendorff preparation at 37 degrees C and constant perfusion pressure of 65 mm Hg). A late protective effect of 7-oxo PGI2 was manifested by an improved recovery of heart function during reperfusion and calcium overload, better preservation of myocardial ATP contents during ischemia and also after calcium overload, as well as by a normalization of the lactate content, otherwise extremely increased during ischemia. Electron microscopic data also supported the above results. The beneficial effect of pretreatment with PGI2 may be explained not only by its vasodilating action, but more by its membrane stabilizing effect with a consequently decreased sodium accumulation, potassium loss, as well as intracellular calcium overload.