The Fawn-Hooded rat (FHR) is a genetic strain that has been extensively studied as a model of primary pulmonary hypertension in adult rats. Based on our recent observations that alveolar number and pulmonary arterial density are reduced in FHRs raised at Denver's altitude, we hypothesized that early abnormalities in pulmonary vascular development contribute to the progression of pulmonary hypertension in the FHR. We found that endothelial nitric oxide synthase (eNOS) protein content was lower in the lungs of fetal, 1- and 7-day-old, 3-week-old, and adult FHRs compared with that in the normal Sprague-Dawley (SDR) and Fischer rat strains, all raised at Denver's altitude. In contrast, lung expression of the endothelial proteins kinase insert domain-containing receptor/fetal liver kinase-1 (KDR/Flk-1) and platelet endothelial cell adhesion molecule-1 (CD31) was not different between strains. Barium arteriograms showed that pulmonary arterial density was reduced in 3-week-old FHRs compared with SDRs. Perinatal treatment of FHRs with mild hyperbaria to simulate sea-level alveolar Po ₂ improved lung eNOS content and pulmonary vascular growth and reduced right ventricular hypertrophy. We conclude that the development of pulmonary hypertension in Denver-raised FHRs is characterized by reductions in lung eNOS expression and abnormal pulmonary vascular growth during the fetal, neonatal, and postnatal periods.