There was conducted an electron microscopy of myocardial samples in mice after extended oral administration of gold nano­particles with the aim of investigation of the effect of gold nanoparticles with a diameter of 2 nm to myocardial ultrastructure. It was shown that oral administration of gold nanoparticles even in the minimal tested dose (10 mg/ml) already at the early stages of the experiment (7 days) leads to a moderately pronounced cytotoxic effect with ultrastructural changes in all components of the myocardium. The severity of degenerative changes increased with the extension of the experiment, but the rough myocar­dial damage (necrosis) was not found at any time of the observation. There were described at unltractructural level changes in microcirculation, interstitium and cardiomyocytes, depending on the concentration of the administered drug and the phase of the experiment. Described degenerative changes in the myocardium caused by the introduction of gold nanoparticles can lead to morphological and functional failure of the heart.