We had previously reported that activation of histamine H(3)-receptors (H(3)R) on cardiac adrenergic nerve terminals decreases norepinephrine (NE) overflow from ischemic hearts and alleviates reperfusion arrhythmias. Thus, we used transgenic mice lacking H(3)R (H(3)R(-/-)) to investigate whether ischemic arrhythmias might be more severe in H(3)R(-/-) hearts than in hearts with intact H(3)R (H(3)R(+/+)). We report a greater incidence and longer duration of ventricular fibrillation (VF) in H(3)R(-/-) hearts subjected to ischemia. VF duration was linearly correlated with NE overflow, suggesting a possible cause-effect relationship between magnitude of NE release and severity of reperfusion arrhythmias. Thus, our findings strengthen a protective antiarrhythmic role of H(3)R in myocardial ischemia. Since malignant tachyarrhythmias cause sudden death in ischemic heart disease, attenuation of NE release by selective H(3)R agonists may represent a new approach in the prevention and treatment of ischemic arrhythmias.