A 6-year-old, castrated male Labrador Retriever was presented for chronic and worsening inappetence, lethargy, exercise intolerance, and exercise-induced collapse. On presentation, the dog had a body condition score of 4.5/5.0, distended jugular veins, and an abnormal jugular fluid wave. A grade V/VI systolic heart murmur was auscultated over the left heart base. Blood analyses revealed a mild increase in alkaline phosphatase (ALP) activity (457 U/L; reference interval [RI] = 20–157 U/L) and a low total thyroxine concentration (tT4 = 8.1 nmol/L; RI = 15.0–48.0 nmol/L). On urinalysis, a specific gravity of 1.038 and 21bilirubinuria were found. Echocardiographic examination showed a 2 4 cm intracardiac mass; it was connected by a small stalk to the infundibular portion of the right ventricular septum and extended outward into the right ventricular outflow tract just proximal to the pulmonic valve. Thoracic radiography and abdominal ultrasonography revealed no additional findings. The patient was diagnosed with right-sided heart failure due to a solitary intracardiac mass that partially obstructed pulmonary outflow during ventricular systole. Samples for cytopathologic evaluation (Figure 1) were obtained from the surgically excised mass. The patient arrested and died shortly after surgical excision. A necropsy was not performed. Figure 1. Impression smear from an intracardiac mass in a dog. Modified Wrights; bars = 50 mm.