A 9-year-old Labrador mixed breed dog, known to suffer from heart disease, was presented. It had recently collapsed on the street. Based on clinical findings and ultrasonography the tentative diagnosis of an atrial rupture with bleeding into the pericardial sac, resulting into a cardiac tamponade, was made. The dog's state of health improved under treatment, including infusion, sedation and analgesia. Therefore pericardiocentesis was unnecessary. The dog was released from hospital the next evening and the medical therapy of its heart disease was continued. The animal died 85 days later.