Аннотация:A most of mature mammals possess by an extra-atrial myocardial structures that are derivatives of an embryonic sinus venosus. These structures are represented by multilayer myocardial sleeves of caval and azygos veins, cardiac tissue in coronary sinus. The aim of present study was to investigate bioelectrical properties of sinus venosus derivatives in mature animals. It has been demonstrated that all mentioned cardiac structures are able to generate various forms of spontaneous firing; aforementioned automaticity is significantly potentiated by adrenergic stimulation.