T he development of pulmonary hypertension in hypoxic newborn calves is associated with a complex pattern of increased tropoelastin and type I procollagen synthesis and deposition by smooth muscle cells in large elastic pulmonary arteries compared to normoxic controls. We examined the possibility that transforming growth factor-beta 1 (TGF-beta 1) may be associated with the production of extracellular matrix protein in this model of pulmonary hypertension. Medial smooth muscle cells in both normotensive and hypertensive vessels, as assessed by immunohistochemistry, were the major source of TGF-beta 1. Staining was confined to foci of smooth muscle cells in the outer media and appeared greater in normotensive than hypertensive vessels. Consistent with the immunohistochemistry, a progressive, age-dependent increase in normotensive pulmonary artery TGF-beta 1 mRNA was observed after birth, whereas TGF-beta 1 mRNA remained at low, basal levels in hypertensive, remodeling pulmonary arteries. These observations suggest that local expression of TGF-beta 1 is not associated with increased extracellular matrix protein synthesis in this model of hypoxic pulmonary hypertension.