Histochemical alterations in the myocardium of rats infected with T. cruzi and killed at regular intervals between 4 and 29 days were studied. The objects of study were: glycogen, phosphorylases, cholinesterases, non-specific esterases, monoamine-oxidase and ATP-ase as marker of muscle fibre activity; alkaline phosphatase as a marker of endothelial activity, NADH and NADPH diaphorases, ubiquinone, cytochrome oxidase, and succinoxidase as a marker of mitochondria in the neurons and the muscle fibres; and esterases, aminopeptidase, and acid phosphatase as lysosomal enzymes. Observations were also made on the staining of RNA and DNA by Acridine Orange. Analysis of the results leads to the following hypotheses: 1. The presence of pseudocysts of leishmanoids in the muscle fibres causes functional changes in the neighbouring myofibrils and mitochondria. 2. the passage of parasites out of the muscle fibres is accompanied by an intense infiltration of phagocytes, diffusing enzymes from which, particularly acid phosphatase and leucyl aminopeptidase, may possibly cause serious damage to the muscle fibres leading to decrease in their myofibrillar and mitochondrial activities. 3. The hypoxia resulting from oedema combined with the action of certain hydrolytic enzymes of the cellular infiltration, might affect certain neurons of the sub-epicardial autonomic ganglia, producing irreversible lesions of chromatolysis and mitochondrial damage, this last evidenced by alterations of some oxidative enzymes. 4. When the parasites disappear, tissue repair begins, conjointly with the regression of the infiltration and the oedema. The function of the muscle fibres gradually recovers except for a prolonged increase of lysosomal activity. The importance of the intense reaction of NADH diaphorase observed in the intracellular stages of the parasites is discussed, together with its possible signifiaance in glycolysis.