A prospective study of neonates with PPHN to determine optimal dosage of nitric oxide for improving oxygenation. This investigation is undertaken to determine the hemodynamic, ventilation, and oxygenation dose-response of inhaled NO (5-80 ppm)in infants with PPHN, to characterize the hemodynamic, ventilation and oxygenation response to withdrawal of inhaled NO, and to identify the characteristics of responders vs. non-responders. This knowledge will increase current understanding of PPHN and determine the mechanism of improved oxygenation with inhaled NO. It will also optimize use of inhaled NO, decrease its toxicity, and thereby decrease morbidity and mortality of infants with PPHN.