SummaryBuckground: As the acute stage of Chagas' myocarditis is rarely detected, little is known about the electrophysiologic characteristics of that stage.Hypothesis: This investigation was undertaken to conduct an electrophysiologic study of the properties of the heart during the acute phase of Chagasic myocarditis.Methods: We studied eight patients who had positive xenodiagnosis, positive mice culture, and positive complement fixation test for Chagas' disease.Results: Trypanosoma cruzi were identified in all of the patients' stained blood samples. Right ventricular endomyocardial biopsies were obtained, evidencing a distinct infiltrate of lymphocytes that confirmed the diagnosis of acute myocarditis. The cardiac dimensions and the Ventricular systolic and diastolic function were preserved in all patients. The electrocardiogram evidenced conduction defects in two patients. The signal-averaged electrocardiogram displayed late potentials in three patients. In the electrophysiologic study, atrial fibrillation or flutter was induced in four patients. When compared with control patients, Chagasic patients were found to have greater values of atrial threshold, A-H interval, and atrioventricular (AV) nodal effective refractory period. The H-V interval was mildly prolonged in two patients, but the dynamic AV nodal conduction was preserved (1 : I conduction during right atrial stimulation at a cycle length of 400 ms) in all the Chagasic patients. The ventricular parameters were within normal limits, and no sustained ventricular arrhythmia could be induced.Conclusions: Patients with mild acute Chagasic myociuditis may suffer from electrical abnormalities and arrhythmias that are more evident at the supraventricular level and the AV junction.