Early restoration of coronary blood flow in acute myocardial infarction with elevation helps reduce the area of myocardial necrosis, prevent dilatation of the ventricular cavity, and reduce the development of arrhythmias and mortality. However, despite modern technologies in STEMI treatment, the development of reperfusion syndrome, in particular, arrhythmias, is possible after the restoration of blood flow in the infarct-related artery, which is a consequence of acute cellular, metabolic and local electrophysiological changes in the myocardium. The aim: is to investigate the long-term effects of reperfusion syndrome on the catabolic/anabolic activity of connective tissue and the processes of connective tissue framework formation in the post-infarction period in patients with myocardial infarction. Materials and methods. The study involved 156 patients with acute myocardial infarction with ST segment elevation (STEMI), and 15 apparently healthy individuals (control of laboratory parameters). The average age of the patients was (57.8±1.2) years. The percent of male patients prevailed (76 %). All MI patients were divided into two groups: Group I (98 people) – MI patients without clinical signs of reperfusion syndrome during revascularization, Group II (58 people) – MI patients with the signs of reperfusion syndrome in the form of rhythm and conductivity disturbances during revascularization of coronary arteries by percutaneous coronarography intervention (PCI) with prolonged stenting of the infarct-dependent coronary artery. Peculiarities of connective tissue metabolism were studied by the content of free and bound oxyproline, glycosaminoglycans, and antibodies to atypical collagens (Ig M and Ig G) in blood serum. Results. The recovery period after myocardial infarction showed an increase in collagen catabolism over collagen synthesizing processes in patients with MI who had reperfusion syndrome. The described patterns were also established by changes in glycosaminoglycans. The study showed an increase in the level of antibodies to atypical collagens. Particularly, the level of Ig M, which was within the norm (0.43±0.18) ng/ml, in patients with MI without reperfusion manifestations was three times higher (p<0.001), and in cases of  combination of MI and reperfusion syndrome – the highest (p<0.001). Conclusions: Changes in collagen-synthesizing processes are observed in patients with MI, with a tendency to increase the processes of catabolism of the connective tissue matrix. The presence of reperfusion syndrome manifestations in MI patients after revascularization aggravates the processes of catabolism, even in the recovery period after MI. Increase in the activity of catabolic processes stimulates fibrosis and formation of rigid matrix and atypical collagen, to which antibodies are synthesized, thus blocking the pathological circuit of restructuring of the heart connective tissue framework, and accordingly its pathological remodeling. The obtained study results contribute to the identification of more sophisticated approaches to revascularization techniques to avoid negative effects of reperfusion syndrome, as well as new effective medications for collagen synthesis correction, which can increase the effectiveness of rehabilitative management of such