The article continues the series of our publications on the problem of nitric oxide (NO) and its cyclic conversion in mammals. This review is held to analysis of nitric oxide role in regulation of cardiovascular system and in alocation of NO-synthases in myocardium. Molecular, biochemical and cytophysiological aspects that linked, with spatial localization of NO-synthases and mechanisms of NO content regulation in myocardium are considered. The results of author's investigations along the cyclic convertion of NO and literature data about compartmentalization of NO-synthases in myocardium are included in this paper. The contradictory and dissimilar facts about regulatory and toxic role of nitric oxide in cardiovascular system are represented.