Calcium antagonistic compounds including physiological ions of K and Mg are supposed to protect the heart against the deleterious effect of calcium overload. Pretreatment with K,Mg-aspartate in a single i.v. dose of 100 mg/kg was performed in dogs 5--10 min prior to isoproterenol chloride injection in a single dose of 7 mg/kh s.c. Administration of K,Mg-aspartate alone in the above-mentioned dose produced reversible changes of mitochondria after 2--6 hr. K,Mg-aspartate pretreatment applied prior to isoproterenol application revealed summarization of both K,Mg-aspartate and isoproterenol-produced changes after 2--6 h without significantly preventing hypoxic-like alterations. However, the final result after 24 h was good improvement of ultrastructure generally resembling metabolic regeneration and/or enhanced metabolic activity.