Young canines born at sea level (SL) and raised for 5 mo at high altitude (HA, 3,800m) followed by return to SL before somatic maturation showed enhanced alveolar gas exchange and diffusing capacity at rest and exercise that persisted into adulthood (McDonough et al., J Appl Physiol, 100:474-81, 2006, Hsia et al., J Appl Physiol, 102:1448-55, 2007). To examine the associated structural response, we quantified lung ultrastructure in male foxhounds raised at 3,800m HA or their littermates raised at SL (n=6 each) from 2.5 to 7.5mo of age. Three years following return to SL, lungs were fixed for morphometric analysis. In HA-exposed animals compared to SL controls, lung volume at a given inflation pressure was higher with enlargement of alveolar ducts and sacs without significant differences in the volumes of alveolar cell components, septal tissue or in alveolar-capillary surface areas. There was a shift towards a lower harmonic mean thickness of the blood-gas diffusion barrier in HA-raised animals. As a control organ, muscle capillary length density of costal diaphragm was significantly higher in HA-raised animals, indicating parallel adaptation in oxygen transport organs. We conclude that in actively growing animals, 5 mo of HA exposure that was discontinued prior to somatic maturation induced acinar remodeling that increased lung compliance and reduced the resistance of blood-gas diffusion barrier to diffusion that persisted into adulthood, but without permanent enhancement of alveolar tissue growth.