In this current study, we used the chronically hypoxic calf model of pulmonary hypertension to evaluate the activity of EC-SOD in a severe neonatal disease model and to better understand the role for extracellular O2.− in the upregulation of Egr-1, a redox sensitive transcription factor implicated in vascular remodeling. Here, EGR1 is linked to pulmonary arterial hypertension.