The authors have analyzed the rhythm of 124 electrocardiograms from 103 autopsied patients with chronic chagasic myocarditis. The most frequent types of arrhythmias were ventricular extrasystoles, atrioventricular block, and bundle branch block. A higher incidence of right bundle branch block was found, but left bundle branch block was more frequently encountered than usually reported. Complete right bundle branch block showed a peculiar distribution of ÂQRS between −60° and −90°, and terminal QRS vectors between −100° and −150°. These orientations seem to be related to the anatomic findings of hypertrophy, dilatation, and apical fibrosis of the ventricles. The presence of right bundle branch block with an upward and left deviation of the ÂQRS, frequently associated with another kind of arrhythmia, is so characteristic in Chagas' disease that if these electrocardiographic signs are found in a young patient with an acquired cardiopathy, they will strongly suggest that we are dealing with chagasic myocarditis, at least in countries where Chagas' disease is endemic. The vectorial analysis of left bundle branch block was without peculiarities, but Q waves in Leads I, aV L, and V 5 even in the presence of complete left bundle branch block was frequently found (45.5 per cent). In 5 of 11 patients showing complete left bundle branch block the ages were 18, 20, 20, 22, and 23 years, respectively. An electrocardiographic diagnosis of bilateral “focal” block was made in 8 cases showing disturbances of the intraventricular conduction and QRS complexes predominantly positive in all precordial leads.