A study was carried out using a model of myocardial ischaemia in the dog after ligation of the left coronary artery to determine the effects of the benzodiazepines, flunitrazepam and midazolam, on the ventricular fibrillation threshold. The fibrillation threshold was measured twice within 15 min and 8 min after ligation. Fifteen minutes after the onset of ischaemic heart attack, 1.2 mg midazolam/kg or 0.25 mg of ischaemia. Their effects on haemodynamics were negligible. It is suggested that both benzodiazepines can be used for increasing electrical stability of the heart in patients with acute myocardial infarction and to inhibit the psychic stress response. Because of its short biological half-life and water solubility, allowing painless intravenous administration, midazolam offers a more flexible approach to pharmacotherapy immediately after acute heart attack according to the patient's current clinical status.