Lung cellular dynamics were examined in growing rats from 4 to 7 wk of age after exposure to 1) room air (general controls), 2) hypobaric normoxia, 3) normobaric hypoxia, 4) hypobaric hypoxia, and 5) room air and restricted food intake (weight-matched controls). Tritiated thymidine ([3H]TdR) incorporation diminished in weight-matched controls. In both hypoxic groups, maximum [3H]TdR incorporation occurred on day 3 in all cells of peripheral alveoli, capillary endothelium of central alveoli, airway walls and epithelium, and arterial wall and endothelium, but maximum [3H]TdR labeling of interstitial and unidentifiable cells of central alveoli occurred on day 5. The percent labeling with [3H]TdR was higher in cells of peripheral alveolar walls than in cells of central alveolar walls. Labeling of the interstitium was higher in hypobaric hypoxic than normobaric hypoxic rats. In hypobaric normoxia, DNA synthetic activity increased in alveolar wall cells, except for capillary endothelium. In hypobaric hypoxia, DNA synthesis occurred primarily because of low O2, but low pressure may also affect cytokinetics. [3H]TdR incorporation is greater and earlier in the alveoli of the peripheral part of the lung than central alveoli, and the cellular response of the various cells types is not synchronous.