Lung morphometry was studied in rats between 4 and 7 weeks of age. The animals were divided into 5 groups: general controls (fed ad libitum), hypobaric normoxic, normobaric hypoxic, hypobaric hypoxic, and weight-matched controls (weight matched to the hypobaric hypoxic group). In both hypobaric and normobaric hypoxia, lung volume, alveolar surface area and total alveolar number increased compared to weight-matched controls. In normobaric hypoxia, mean linear intercept, mean chord length of alveoli increased and number of alveoli/unit volume decreased compared to weight-matched animals. In hypobaric hypoxia, only mean chord length increased. Dysanaptic index decreased in both. In hypobaric normoxia, alveolar size and lung volume diminished compared to general controls. Lung growth was impaired in weight-matched controls without affecting airspace dimensions. Hypobaric and normobaric hypoxia increase lung growth overcoming nutritional effects but is dysanaptic. Lung growth in hypobaric hypoxia is mainly determined by low oxygen but low pressure may also produce subtle structural alterations.