Persistent pulmonary hypertension of the newborn (PPHN) often requires extracorporeal membrane oxygenation (ECMO), during which time pulmonary vascular resistance gradually declines. Nitric oxide (NO) is a recently recognized pulmonary vasodilator, but its role in PPHN is unknown. We tested the hypothesis that the concentrations of the urinary metabolites of NO, i.e. nitrite and nitrate, are reduced in patients with PPHN and increase during ECMO as the PPHN resolves. Eight newborn infants with PPHN on ECMO were studied. Daily urinary concentrations of nitrite/nitrate were measured. We found that mean urinary concentrations of nitrite/nitrate were lower in patients with PPHN than in 47 controls without pulmonary disease (p < 0.005). Urinary nitrite/nitrate concentration showed an initial increase after initiation of ECMO. However, a decrease to concentrations still lower than controls occurred on the day before decannulation. We conclude that intrinsic NO production is significantly lower in patients with PPHN than in controls but increases with oxygenation. We speculate that decreased urinary NO metabolite concentrations imply a role for NO deficiency in the pathogenesis of PPHN.