Organic nitrates have frequently been used for the treatment of patients with congestive heart failure (CHF). Nitrate tolerance has been identified as a major limitation of this therapy preventing a continuous effect. In the past several years, an effort has been made to develop strategies to prevent nitrate tolerance. Recent animal experimentation has demonstrated the prevention of nitrate tolerance with a concomitant administration of hydralazine. A later study demonstrated similar results in a patient with chronic CHF. The use of oral hydralazine (75 mg qid) resulted in prevention of early development of nitrate tolerance and attenuation of nitrate mediated hemodynamic effects which occurred within the first several hours after initiation of nitroglycerin therapy in the control group not receiving hydralazine. Recent data obtained in in vitro studies have demonstrated enhancement of vascular superoxide production secondary to exposure to nitrates which may be an important mechanism leading to nitrate tolerance. These same studies demonstrated a strong antioxidant effect of hydralazine reducing superoxide formation and preventing nitrate tolerance. Both animal and human experiments have demonstrated prevention of nitrate tolerance with a concomitant use of hydralazine. In vitro studies suggest that hydralazine, by virtue of its antioxidant effect, prevents the nitrate-mediated formation of vascular superoxide, and thus prevents nitrate tolerance. These data provide an explanation for the benefit demonstrated in the V-HeFT studies with the combination of isosorbide dinitrate and hydralazine in the treatment of patients with chronic CHF, and provide a support for the use of this therapeutic regimen in the treatment of patients with chronic heart failure.