<p>The aim of this work was to study changes in the spectral characteristics of the heart rhythm of experimental animals under the influence of the class I antiarrhythmic drug ethacizine hydrochloride and its supramolecular complexes with glycyrrhizic acid (GA) and its monoammonium salt (MASGA) in various mole ratios (2:1, 4:1, 8:1). It was shown that the complexes GA-ethacizine HCl (2:1) and MASGA-ethacizine HCl (4:1) had stronger negative inotropic effects in which Na⁺ channels of cardiomyocytes played an important role as compared to ethacizine and the other complexes. At the same time, these complexes promoted increased excretion of Ca²⁺ ions through the Na⁺/Ca²⁺ exchanger and a decreased level [Ca²⁺]_i level by blocking Na⁺ channels and reducing the content of Na⁺ ions in cardiomyocytes, resulting in a decrease in the contractile activity of the heart muscle. It was determined during the studies that the studied complexes had a more pronounced antiarrhythmic effect than ethacizine, especially the MASGA-ethacizine HCl (4:1) complex, which may become a promising antiarrhythmic drug as a result of further studies.</p>