Although ischemic heart disease is invariably associated with marked activation of sympathetic nervous system, elevated levels of circulating catecholamines and lethal ventricular arrhythmias, the mechanisms of arrhythmogenesis due to myocardial ischemia are not fully understood. Since catecholamines are known to produce stimulatory effects in the heart mainly by acting on β₁-adrenoceptors, this study was undertaken to test the involvement of these receptors in the development of arrhythmias due to myocardial infarction (MI) induced upon occluding the left coronary artery in rats for a period of 2 h. The animals were treated with or without atenolol (20 mg/kg; daily), a selective β₁-adrenoceptors blocker, for 14 days before inducing MI. No alterations in the number of MIinduced episodes and incidence or duration of different types of arrhythmias were observed. In fact, the incidence of trigemines and reversible ventricular fibrillation due to MI were significantly increased in the atenolol-treated animals. These observations support the view that the activation of β₁-adrenoceptors may not be exclusively involved in the development of arrhythmias during the occurrence of ischemic heart disease and other mechanisms can underlie the electric instability of such damaged heart.