Nitric oxide (NO) is thought to be a primary mediator of the reduction in pulmonary vascular resistance which occurs in the newborn period. L-arginine is the precursor for the formation of nitric oxide in the pulmonary endothelium. Low serum arginine levels have been reported in infants with persistent pulmonary hypertension of the newborn (PPHN). We infused a single L-arginine dose of 500 mg/kg over 30 min to 5 consecutive infants with PPHN. Ninety minutes after infusion we observed an associated rise in PaO2 of 37 to 84 mm Hg and, in 4 of 5 infants, a reduction in oxygenation index (OI) of 33-50% over the 5-hour period following infusion. Infusion was not associated with adverse effects. These observations suggest that L-arginine administration may be an effective therapeutic alternative in infants with PPHN.