Background. Pulmonary hypertension secondary to valvular heart disease is a cause of acute right heart failure during valve replacement operations. This study compares the hemodynamic effects of intravenous use of iloprost and nitroglycerin in patients with pulmonary hypertension undergoing valvular replacement surgery. We sought to determine the acceptable doses of these medications for use in surgery to decrease mean pulmonary artery pressure to <30 mmHg without causing systemic side effects. The plasma nitric oxide levels that were obtained from pulmonary mixed venous blood have been compared to demonstrate the difference in the action mechanism of these drugs.