Myocardial fibrosis was formerly chiefly ascribed to healed infarcts and to interstitial myocarditis. Recently, however, more attention has been paid to myocytolysis as another possible source of fibrosis (SCHLESINGER & REINER, MAGARINOS TORRES). Besides focal myocytolysis of the myocardium well recognized in later years, another type-diffuse myocytolysis-is reported in this paper. In Chagas' heart disease fibrosis of the myocardium is largely the consequence of diffuse myocytolysis. In its chronic stage (figs. 3-11) which has been often mistaken for interstitial myocarditis, diffuse myocytolysis while not specific is always found and often extensive in Chagas' heart disease and this circumstance should be taken into account when attempted the micorscopic diagnosis of such condition. Its acute stage (figs. 13 and 14), is seldom found suggesting a slow progress for this process, as only a few myocardial fibers are attained at the same time. This point is worth considering when performing its study with the electron microscope. C. Magarinos Torres' lesion (figs. 15, 16 and 17) is likely not related to myocytolysis although both changes probably recognize identical pathogeny: metabolic disturbances in myocardial fibers induced by local anemia. The need for arterial blood supply is likely diminished in the areas of marked diffuse myocytolysis on account of extensive destruction of myocardial cells and this possibly accounts for collapse of small arterial branches (figs. 18, 20 and 22) occasionally found. The original stroma with preserved blood capillaries in the areas of the so-called "fibrosis" (figs. 8, 9, 11 and 12) distinguishes diffuse myocytolysis from chronic interstitial myocarditis. Marked dilatation of such capillaries (fig. 12) demonstrates the high degree of congestive cardiac insufficiency found in fatal cases of Chagas's heart disease and is indicative of a vicious circle: passive hyperemia, local anemia, metabolic disturbances in myocardial fibers, myocytolysis. Data here reported reinforce the concept that in Chagas' heart disease vascular changes intimately associated to a specific myocarditis are at work.