Chronic Chagasic heart disease has many features characteristic of other congestive cardiomyopathies, including ventricular and atrial chamber enlargement, hypertrophy, focal scarring, and mural thrombi. Histologically, there is often lymphocytic inflammation, spotty necrosis, and few parasites. Although immunologic mechanisms have been invoked to explain the development of myocardial degeneration, there have been suggestions that the focal alterations in the heart are secondary to abnormalities of the coronary microcirculation. Based on work from our laboratories which has demonstrated microvascular hyperreactivity in several other models of congestive cardiomyopathy, we investigated whether the cardiac microcirculation of mice acutely infected with Trypanosoma cruzi was also abnormal. We perfused animals at 15–17 days post-infection with silicone rubber which fills the arterioles, capillaries, and venules of the beating heart. After clearing the tissue, we observed numerous areas of focal vascular constriction, microaneurysm formation, dilatation, and proliferation of microvessels which were not present in control animals. These lesions were similar to those we have observed in other congestive cardiomyopathies. Since at this stage of infection there is minimal cardiac degeneration or fibrosis, the presence of these vascular lesions even early in Chagas' disease, may be significant for the pathogenesis of focal myocardial damage. These observations during acute infection provide additional support for the suggestions of others that the myocardial microcirculation is abnormal in Chagas' disease.