Hypoxia-induced PPHN is characterized by sustained vasospasm and increased thromboxane:prostacyclin ratio. We previously demonstrated that moderate hypoxia induces myocyte thromboxane hypersensitivity. Here, we examine thromboxane prostanoid receptor (TP-R) localization and kinetics following hypoxia to determine mechanism of hypoxic TxA hypersensitivity. Primary cultured neonatal pulmonary artery myocytes were exposed to 10% O₂ (HM) or 21% O₂ (NM) for 3 days. PPHN was induced in neonatal piglets by in vivo exposure to F_iO₂ 10% for 3 days. TP-R was studied in whole lung sections from pigs with hypoxic-PPHN and age-matched controls; intracellular localization was studied by immunocytochemistry. Myocyte TP-R saturation binding was studied using [H³]-SQ29548, and competitive binding after co-incubation with U46619. Phosphorylation and coupling were examined in immunoprecipitated TP-R. We report distal propagation of TP-R expression in PPHN, extending to pulmonary arteries <50µm. In HM, intracellular TP-R tranlocates to a perinuclear region, mirroring altered ER morphology. Hypoxic TP-R kinetics reveal a decreased K_d and B_max. Additionally, in hypoxia [H³]-SQ29548 is displaced at lower concentration of U46619 than in normoxia, suggesting increased agonist affinity. Serine phosphorylation of HM TP-R is significantly decreased compared with NM; this difference correlates with increased Gq coupling in hypoxia, and is ablated by incubation with PKA. We conclude that the TP-R is normally desensitized in the neonatal pulmonary circuit by PKA-mediated regulatory phosphorylation, decreasing ligand affinity and coupling to Gq; this protection is lost following hypoxic exposure. Appearance of TP-R in resistance arteries after development of hypoxic PPHN may contribute to increased pulmonary arterial pressure.