Increasing data suggest that oxidative stress is involved in the pathophysiology of pulmonary hypertension. Several antioxidant systems regulate the levels of oxidant species, and of primary interest are the superoxide dismutases and catalase. However, little is known about the expression of antioxidant enzymes (AOEs) during the development of pulmonary hypertension. This study uses our lamb model of increased post-natal pulmonary blood flow, secondary to in utero aorto-pulmonary graft placement (shunt lambs), to investigate the expression patterns and activities of AOEs during the early development of pulmonary hypertension. Protein levels of catalase, SOD1, SOD2, and SOD3 were evaluated by Western blot, and the activities of catalase and SOD were also quantified. In control lambs, protein expression and activities of catalase and SOD2 increased postnatally. However, SOD1 and SOD3 protein levels were unchanged. In shunt lambs, catalase, SOD1, and SOD2 protein levels all increased over the first 8 weeks of life. However, SOD3 was unchanged. In addition, activities of catalase and SOD2 also increased. Compared to control lambs, catalase and SOD2 protein levels were decreased in 2 week-old shunt lambs and this was associated with increased levels of hydrogen peroxide (H₂O₂) and superoxide. Developmentally superoxide- but not H₂O₂-levels significantly increased in both shunt and control lambs with levels being significantly higher in Shunt- compared to control-lambs at 2- and 4- but not 8-weeks. These data suggest that the antioxidant enzyme systems are dynamically regulated postnatally, and this regulation is altered during the development of pulmonary hypertension secondary to increased pulmonary blood flow.