Apelin is a new cytokine with proven functions in the cardiovascular system. It is derived from a single gene and has several isoforms that differ in polypeptide chain length and biological activity. In its biological effect participates APJ-receptor. Apelin shows structural similarity to the RAS. The highest concentrations of apelin mRNA and its receptor was observed in the endocardium and the endothelium of smooth muscle cells of blood vessels, which facts relate to its inotropic positive action and hypotensive effect. The positive role of apelin in the early stages of cardiac failure is emphasized. Furthermore, the negative correlation between its concentration and the value of systolic blood pressure in patients with essential arterial hypertension, gained attention. Its participation in coronary artery disease is also called on. Moreover apelin affects endothelial cell proliferation and vascular formation. Apelin also plays the role in carbohydrates utilization by reducing insulin resistance and improving glucose tolerance in the early stage of development of type 2 diabetes. Due to its participation in modification of insulin action, the effect of apelin is being compared to the role of adipocytokines associated with the pathogenesis of obesity and insulin resistance, such as, wisfatin, adiponectin, leptin, resistin, and TNF-α. Apelin also influences the secretion of several hormones and it is a marker of endocrine cells in the gastrointestinal tract. More elaborated knowledge of the mechanism of apelin action in the human and its function in various systems, requires further research that may contribute to the development of therapeutic effects, especially in cardiovascular diseases.