A 15-year-old donkey gelding was referred to the Equine Clinic of the Freie Universität Berlin because of acute onset of anorexia, bradycardia and multiple syncopes. The clinical examination revealed an irregular heartbeat (11-18 bpm) and syncopes. A continuous ECG revealed absence of P-waves and an irregular ventricular escape rhythm. An echocardiography revealed a pericardial effusion with reduction of left and right ventricular function. Laboratory abnormalities revealed increased troponin I, alpha hydroxybutyrate dehydrogenase and symmetric as well as asymmetric dimethylarginine. Increased concentrations of the cardiac glycoside digitoxin was evident in serum. Clinical findings were consistent with myocarditis with pericardial effusion and atrial standstill. Because of increasing severity of symptoms and grave prognosis the donkey was humanely euthanized. Histopathological examination of the cardiac atria including the sinoatrial node revealed severe, subacute, diffuse, suppurative myocarditis.