Assessment of the state of exercise tolerance (ET) after the transfer of COVID-19 is necessary to determine the long-term consequences of the coronavirus disease. Purpose - to assess the state of ET and analyze changes in myocardial reserve indicators in children who have experienced COVID-19 of varying degrees of severity. Materials and methods. 155 school-age children took part in the study. 120 children with a confirmed diagnosis of COVID-19 in the anamnesis and a period after COVID-19 of more than 12 weeks (main group) and 35 healthy children who did not suffer from COVID-19 (control group). The Group I (n=49) included children who contracted mild COVID-19; the Group II (n=40) - children who suffered from COVID-19 of moderate severity, the Group III (n=31) - children who contracted COVID-19 with a severe course. General clinical, instrumental (cycle ergometry), and statistical research methods were used. Results. In the main group, the average value of maximum oxygen consumption (VO2max) was 26.86±6.69 ml/kg/min, and in the control group - 38.29±5.54 ml/kg/min (p<0.001). The analysis of this indicator in the Groups I, II, and III showed that transferred COVID-19 affects the level of ET in boys regardless of the severity of COVID-19 (p<0.01). In girls, reduced ET was determined in the case of moderate and severe course of COVID-19. The analysis of myocardial reserves indicators showed that among children who fell ill with COVID-19, the worst indicators were observed in the Groups II and III (p<0.01). Conclusions. Transmitted COVID-19 affects the state of ET and indicators of myocardial reserves in children in the period more than 12 weeks after COVID-19. The level of changes in the state of ET, values of the cardiac and vascular component of physical load support, and myocardial reserves depend on the degree of severity of the transferred COVID-19, which is confirmed by correlations of varying strength between these indicators and the degree of severity of COVID-19. The research was carried out in accordance with the principles of the Helsinki Declaration. The study protocol was approved by the Local Ethics Committee of the participating institution. The informed consent of the patient was obtained for conducting the studies. No conflict of interests was declared by the authors.