Objective. To study the morphological changes of the cardiac conduction system and the working myocardium in HIV-infected drug-dependent patients who died from infective endocarditis and generalized tuberculosis, and to compare these data with the results of in vivo electrocardiographic studies. Material and methods. Morphological and electrocardiographic data of 59 HIV-infected drug-dependent patients who died from infective endocarditis (37) or generalized tuberculosis (22) have been analized. The study of the morphological structure of the cardiac conduction system was performed according to the modified method proposed by Yu.G. Parkhomenko et al., that includes the preparation and microscopy of micropreparations containing elements of the cardiac conduction system and other heart structures. Results. Morphological changes in the cardiac conduction system (degenerative, inflammatory, necrobiotic, circulatory disorders, myocardiosclerosis) were found an average of 10–20% less than in the working myocardium and amounted to 70% of cases of infective endocarditis and 82% – generalized tuberculosis, at that inflammatory changes prevailed in infectious endocarditis and dystrophic changes – in generalized tuberculosis. When analyzing electrocardiograms non-specific changes dominated in both groups characterize the state of the working myocardium, which is confirmed by histological data: the most severe morphological changes were observed in the working myocardium at endocarditis involving the left heart and generalized tuberculosis. Сonduction disorders (blockage of the right and left His bundle branches, atrioventricular conduction slowing) were recorded on an electrocardiogram in 14% of deaths from infective endocarditis and in 23% of deaths from the generalized tuberculosis and morphologically corresponded to maximum changes of certain elements of the cardiac conduction system. Both morphological and electrocardiographic data in both groups showed a predominant involvement of the right and left His bundle branches, more rarely the atrioventricular node and bundle were affected, and the most rare was the lesion of sinoatrial node. Special attention is given to the detection of elongated Q–T interval on electrocardiograms in both groups (9 and 11% of cases at infective endocarditis and generalized tuberculosis, respectively), which is the predictor of life-threatening arrhythmias and refers to HIV-associated cardiovascular complications. Histologically the cases of elongated Q–T interval were characterized by non-specific changes in the form of edema and dystrophy of working cardiomyocytes. Conclusion. The basis of electrocardiographic changes at infectious endocarditis and generalized tuberculosis are morphological changes in the cardiac conduction system and working myocardium, however, the frequency of detection of the pathology of heart conduction system structure is several times higher than of its functional manifestations, which indicates its high compensatory capacity. The pattern in the defeat of various departments of the conduction system of the heart is due to its anatomical and topographical features.