A 12-year-old female spayed West Highland White Terrier was referred to the Cornell University Hospital for Animals Cardiology Service for fmther evaluation of a several year history of lethargy and exercise intolerance. The primary veterinarian noted a marked bradycardia. The Holter monitor revealed pauses in cardiac rhythm up to 8 seconds long, but no episodes of collapse, seizures, or loss of consciousness. Physical examination at Cornell revealed bradyarrhythmia and a honking cough. Electrocardiography displayed sinus bradycardia with intermittent sinus pauses suggestive of Sick Sinus Syndrome. Bilateral atrioventricular valve regurgitation, pulmonary hypetiension, aortic insufficiency, and a persistent left cranial vena cava were observed on echocardiography. Left-sided cardiomegaly was noted on thoracic radiographs. Serum cardiac troponin I levels were normal. Based on the dog's size, difficult vascular access, and persistent left cranial vena cava, the decision to place an epicardial pacemaker was made. This presentation will discuss the diagnosis and treatment of Sick Sinus Syndrome.