Diazepam did not alter the rate or rhythm in III patients with atrial fibrillation. In 3 out of 38 patients with atrial flutter the use of diazepam was followed by an increase in atrioventricular block. Diazepam restored sinus rhythm in I patient with atrial tachycardia (total 7 patients) and in I patient with ventricular tachycardia (total 8 patients). Experimentally in the dog diazepam raised the electrically-induced ventricular tachycardia threshold significantly. Pretreatment with diazepam did not alter significantly the dose of strophanthidin K required to induce ventricular tachycardia in the dog. The value of diazepam as a cardiac anti-arrhythmic agent should be further assessed in a controlled clinical trial, especially in patients with acute myocardial infarction.