The consequences of acute inflammatory damage and chronic repair in the conducting system of the canine heart were observed after experimental Trypanosoma cruzi infection of young dogs. Acute infection lasted for 18-25 days and caused hyaline and lytic necrosis of the atrio-ventricular conducting fibres, the sinus node and the neurons in the cardiac para-sympathetic ganglia. Dogs that survived acute disease were examined after periods from 8 months to 3 years. Three main types of healing were found in the conducting system: fibrosis (diffuse and/or focal), sclero-atrophy and fatty infiltration. No signs of active chronic inflammation were present, although small focal residual accumulations of mononuclear cells were present in some animals. No evident electrocardiographic alterations were elicited by the presence of such cicatricial lesions. The animal model appears worthwhile for the study of the healing process in conducting tissue of the heart and for the better understanding of the asymptomatic phase of T. cruzi infection in man.