Newborn rats were exposed to 10% O2 from 24 h to 6 days after birth, then returned to normoxia and examined at 50 days of age, i.e., after reaching sexual maturity. Despite the important impairment in somatic growth during hypoxia, at 50 days body weight and nose-tail length were as in control rats never exposed to hypoxia. Hypoxic rats had a bigger chest, with larger anteroposterior diameter, larger surface area of the muscle component of the diaphragm, and heavier and more expanded lungs. None of these structural changes were observed in a third group of rats, which were exposed for 6 days to hypoxia between 35 and 42 days of age, i.e., at a much more advanced stage of postnatal development. In addition, hypoxic rats had higher compliance of the respiratory system and of the lung and lower total pulmonary resistance than control rats. Frequency dependence of compliance was not different. We conclude that in the rat the structural changes induced by neonatal chronic hypoxia are not resolved by the return to normoxia but persist at least until postpuberty with modifications of the mechanical properties of the respiratory system.