Under successive coperfusion of the arterial vascular rings and right atrial trabecula an examination of effects of endothelium-derived NO on reactions of vessel and myocardial trabecula recipient has been done. During arterial rings contraction a relaxing factor was released into solution. By means of chemical de-endothelialization, NO-synthase inhibition and L-arginine the role of endothelium as a source of modulated substances and the role of NO as a main dilated substance for recipient preparations were shown. The direct effect of endothelium-derived NO on a tension and reactivity of myocardial trabecula has been demonstrated.