Myocarditis in dogs is an uncommon disease process that is difficult to diagnose and treat1• In this case, an 8 year old castrated male Labrador Retriever presented with a supraventricular tachycardia (300bpm) that was initially diagnosed as a ventricular tachycardia by the referring veterinarian. Electrocardiogram (ECG) showed an incessant, regular, narrow QRS complex supraventricular tachycardia (SVT) with an electrocardiographic diagnosis of atypical atrial flutter. Daily repeated echocardiograms suggested an mrhythmia-induced dilated cardiomyopathy that appeared to mildly improve after rate confrol with sotalol administration. Based on elevated troponin and SVT, acute/subacute myocarditis was presumptively diagnosed. Definitive diagnosis using endomyocardial biopsies and treatment with catheter ablation were not available at Cornell at the time. Due to lack of owner compliance and recuning SVT, the dog's clinical status declined and he was euthanized two weeks following discharge from the hospital.