Blood and lymphatic vessels in patients with rheumatic heart lesions in active and inactive phases of rheumatism were studied electron-microscopically and electron-histochemically. In the active phase of the disease there were revealed dystrophic changes in endothelial and smooth-muscle cells, fragmentation of the inner elastic membrane, the splitting of collagenic fibrils of media and adventita. In the invactive phase dystrophic changes in cells were not observed in media and adventitia, an increase in the amount of collagenic structures was noted, and in hyalinized walls of arteries there were considerable aggregations of material identical to the substance of the basal membranes of the endothelium and smooth-muscle cells. These changes were combined with evidences of elevated permeability (extravasates) and vulnability to injuries (micro-hemorrhages) in the micro-blood vessels of the myocardium.