Structural reorganization of the myocardium in response to antitumor agents (cyclophosphamide, betulonic acid and its amide) was studied. Cardiotoxic effects of these chemicals manifested in cardiomyocyte contracture and lytic injuries and by significant hemodynamic disorders. The most pronounced lytic and necrobiotic changes in cardiomyocytes were detected after injection of cyclophosphamide followed by betulonic amide; this led to a more pronounced decrease in heart weight as a result of a decrease in total cardiomyocyte count. Antitumor drugs differently changed the ratio of mono- to binuclear cardiomyocytes, which differ by their regeneratory and compensatory adaptive potential.