Objective: To study immunohistochemical and electron-microscopic features of secretory cardiomyocytes in experimental myocardial infarction (MI). Materials and Methods: Totally 15 hearts of dogs with experimental MI were studied. For electron-microscopic study materials were collected from left atrium, and its several parts (appendix, lateral and frontal walls). For immunohistochemical analysis of atrial natriuretic factor (ANF) incubation in monoclone immune serum was used (standard monoclonal serum and primary mice antibodies, Immunon). For statistic processing we used Chi-square test (criterion of Pearson agreement). Results: Immunohistochemical and electron-microscopic investigation after 24 hours from experimental myocardial infarction indicated increase of specific activity of secretory cardiomyocytes and after 48 hours decrease of secretion of ANF while cardiomyopathy appears. After 72 hours, blockade of ANF secretion with decompensation of secretory cardiomyocytes occurred. Conclusion: Imunohistochemical investigations and analysis of submicroscopic structures of secretory cardiomyocytes after experimental MI showed, that cells were functionally active 24 hours after myocardial infarction with further (48 hours and 72 hours of MI) decrease in amount and impairement of activity.