Endothelin-1 (ET-1) mediates hypoxia-mediated pulmonary vascular remodeling (HPVR), and endothelin-A receptor (ET-AR) blockade prevents HPVR in newborn mice. Our objective was to determine postnatal effects of chronic hypoxia and/or ET-AR blockade on lung ET-1, ET-AR, ET-BR, and vascular collagen and elastin. Newborn C57BL/6 mice (n= 68/gp) given either BQ610 (ET-AR blocker) or vehicle were exposed to air or hypoxia (12% O2) from birth for 1, 3, or 14 d. Lung ET-1 was assessed by ELISA, and ET-AR and ET-BR by immunohistochemistry. Vascular collagen and elastin were assessed by quantitative image analysis. ET-1, ET-AR, ET-BR, collagen I and III, and tropoelastin mRNA levels were assessed by real-time quantitative RT-PCR. We observed that: 1) hypoxia attenuated the normal postnatal decrease in ET-1 and collagen content; 2) ET-AR blockade reduced collagen independent of O2; 3) hypoxia increased elastin mRNA expression and attenuated the normal postnatal decrease in elastin content; and 4) BQ610 reduced elastin mRNA but not elastin content. We conclude that, in neonatal mice, hypoxia attenuates normal postnatal decreases in ET-1, vascular collagen, and elastin. ET-AR blockade reduces collagen fiber area but not mRNA, and does not decrease elastin despite reducing its expression. Abbreviations: ECMextracellular matrix; ET-ARendothelin-A receptor; ET-BRendothelin-B receptor; ET-1endothelin-1; HPVRhypoxia-induced pulmonary vascular remodeling. © International Pediatrics Research Foundation, Inc. 2007. All Rights Reserved.