Male Wistar rats were inoculated intraperitoneally with approximately 2 × 106Trypanosoma cruzi Y strain blood forms. On days 7, 50, and 185 after inoculation, the animals were killed, and the right cervical vagus nerve was dissected, postfixed in 1% osmium tetroxide, and embedded in epoxy resin (Araldite). Semi-thin transverse sections were stained with 1% toluidine blue, examined by light microscopy, and photographed. An image analysis system was used to measure the area and diameter of each nerve and each fiber visible on the photomicrographs. Inoculated animals killed on days 7 and 185 after inoculation did not present morphologic or morphometric alterations of the vagus nerve. Inoculated animals killed on day 50 after inoculation presented several degrees of structural disorders in the myelin sheaths compared with control animals. The morphometric data demonstrated that the diameter of the myelinated fibers was generally increased in inoculated animals killed on day 50 after inoculation. These results suggest that experimental Chagas' disease in rats causes myelin damage and axonal swelling of the myelinated fibers of the vagus nerve, and that this injury to the vagus nerve may be important for a better understanding of the pathogenic mechanisms of the cardiac and digestive alterations caused by T. cruzi.