We previously showed that fetal lambs whose ductus arteriosus is ligated prenatally will have persistent pulmonary hypertension at birth. We investigated the effect of inhaled nitric oxide on the pulmonary circulation in this animal model. The ductus arteriosus of six fetal lambs was ligated at 126 days of gestation. The lambs were delivered and studied at 136 days of gestation. Mechanical ventilation was maintained at a fraction of inspired oxygen of 0.80. Nitric oxide gas was administered at five different concentrations (6, 12, 25, 50, and 100 ppm) for 5-minute periods separated by 10-minute periods of ventilation without nitric oxide. Inhaled nitric oxide caused dose-dependent decreases in pulmonary arterial pressure and vascular resistance and dose-dependent increases in pulmonary blood flow without affecting systemic arterial pressure. Thus pulmonary arterial pressure decreased from equal to aortic pressure to less than aortic pressure. At the highest dose, mean pulmonary arterial pressure decreased by 27%±2%, pulmonary blood flow increased by 86%±6%, and pulmonary vascular resistance decreased by 59%±4%. Nitric oxide also caused dosedependent increases in systemic arterial oxygen tension and in the saturation of hemoglobin with oxygen. Partial pressure of arterial oxygen increased from 43±16 mm Hg at baseline to 185±72 mm Hg at the highest dose; saturation increased from 74%±8% to 96%±2%. In our model of persistent pulmonary hypertension of the newborn, inhaled nitric oxide selectively dilates the pulmonary circulation, thereby improving systemic arterial oxygenation. Nitric oxide is a promising new treatment of persistent pulmonary hypertension of the newborn. © 1993 Mosby-Year Book, Inc.