Congenital cardiac defects associated with increased pulmonary blood flow (Q_p) produce pulmonary hypertension. We have previously reported attenuated endothelium-dependent relaxations in pulmonary arteries (PA) isolated from lambs with increased Q_p and pulmonary hypertension. To better characterize the vascular alterations in the nitric oxide-superoxide system, 12 fetal lambs underwent in utero placement of an aortopulmonary vascular graft (shunt). Twin lambs served as controls. PA were isolated from these lambs at 4–6 wk of age. Electron paramagnetic resonance spectroscopy on fourth-generation PA showed significantly increased superoxide anion generation in shunt PA that were decreased to control levels following inhibition of nitric oxide synthase (NOS) with 2-ethyl-2-thiopseudourea. Preconstricted fifth-generation PA rings were relaxed with a NOS agonist (A-23187), a nitric oxide donor [ S-nitrosyl amino penicillamine (SNAP)], polyethylene glycol-conjugated superoxide dismutase (PEG-SOD), or H₂O₂. A-23187-, PEG-SOD-, and H₂O₂-mediated relaxations were impaired in shunt PA compared with controls. Pretreatment with PEG-SOD significantly enhanced the relaxation response to A-23187 and SNAP in shunt but not control PA. Inhibition of NOS with nitro-l-arginine or scavenging superoxide anions with tiron enhanced relaxation to SNAP and inhibited relaxation to PEG-SOD in shunt PA. Pretreatment with catalase inhibited relaxation of shunt PA to A-23187, SOD, and H₂O₂. We conclude that NOS catalyzes the production of superoxide anions in shunt PA. PEG-SOD relaxes shunt PA by converting these anions to H₂O₂, a pulmonary vasodilator. The redox environment, influenced by the balance between production and scavenging of ROS, may have important consequences on pulmonary vascular reactivity in the setting of increased Q_p.