1. The resting electrocardiogram was obtained from 25 Trypanosoma cruzi -infected rats 30 days after infection (phase I). The resting electrocardiogram was abnormal in 12 (group I) and normal in 13 (group II) animals. Nineteen similar but non-infected animals served as controls. Both the resting electrocardiogram and the ajmaline test were performed 120 and 350 days after infection (phases II and III, respectively). 2. With regard to the resting electrocardiogram of group I animals, left axis deviation was found in 10 of 12 (83%) in phase I, one of 12 (8%) in phase II ( P <0.05) and in none in phase III ( P <0.05). An intraventricular conduction delay was found in four of 12 (33%) rats in phase I, two of 12 rats (16%) in phase II ( P >0.05) and six of 12 rats (50%) in phase III ( P >0.05). The ajmaline test was abnormal in nine of 10 (90%) rats of group I with normal resting electrocardiogram in phase II, and in three of six (50%) animals in phase III ( P >0.05). 3. An intraventricular conduction delay was found in the resting electrocardiogram of one of 13 (7%) rats of group II in phase III. The ajmaline test was abnormal in one of 13 (7%) rats in phase II and in one of 12 (8%) rats in phase III. 4. No control rat showed pathological changes. Eight of 12 (66%) rats of group I and two of 13 (15%) rats of group II showed histopathological alterations consisting of mononuclear cell infiltrate, cellular vacuolation and myocardial necrosis. 5. We conclude that different abnormal patterns may be seen in the resting electrocardiogram in the same T. cruzi -infected rat over time. The QRS axis changes predominate during acute and intraventricular conduction disturbances in the chronic stage of Chagas' heart disease in rats.