Hypoxia-inducible factor (HIF)-1α is associated with hypoxia-induced pulmonary hypertension (HPH) in adults. In the present study, the expression levels of HIF-1α, endothelin (ET)-1 and adrenomedullin (ADM) were analyzed during HPH in neonates. In total, 96 newborn rats were subjected to hypoxia or normoxia for 3, 5, 7, 10, 14 or 21 days (n=8 per subgroup). HIF-1α, ET-1 and ADM expression levels were measured by quantitative polymerase chain reaction. In addition, the intima-media thickness/external diameter ratio (MT%) and medial wall cross-sectional area/vessel total cross-sectional area ratio (MA%) were calculated to evaluate pulmonary vascular remodeling. The mean pulmonary arterial pressure (mPAP) increased with exposure to hypoxia. Furthermore, the expression levels of HIF-1α, ET-1 and ADM in the lungs were shown to increase after three and five days of hypoxia, while the MT% and MA% increased after seven days of hypoxia, as compared with the controls (P<0.05). Therefore, the expression of HIF-1α, ET-1 and ADM is upregulated in the lungs of newborn rats during early HPH. At later stages, the mPAP increases, vascular remodeling occurs and HIF-1α, ET-1 and ADM expression levels restore to normal levels.