Postnatal lung development was examined in rats born with smaller than normal lungs after either prenatal exposure to glucocorticoid or to an inhibitor of collagen synthesis. At birth, treated animals had lower than normal lung weights, lung to body weight ratios, hydroxyproline (HYP) levels, total DNA; and rates of DNA synthesis. Rats exposed to steroid showed a rapid recovery in growth during the normal postnatal cell proliferative phase from 4 to 11 days, though collagen levels did not return to normal until 3 weeks of age. Rats exposed to a prenatal proline analog showed a much slower rise in lung weight and total DNA, and these levels were still much below normal at 2-3 weeks when the cell proliferation phase was completed. Levels of disaturated phosphatidylcholine were significantly below normal up to 11 days, whereas total HYP was significantly reduced and less fibrillar collagen was seen in the lung throughout the study. The results indicate that the smaller but mature lungs at birth after antenatal steroid show a growth rebound and quickly become structurally normal. In contrast, inhibition of fibroblast growth and collagen deposition produces small lungs at birth, which continue to show inhibited growth and development at least up to 3 weeks of age, when the cell division phase is over.