Prematurely born babies are often treated with glucocorticoids. We studied the consequences of an early postnatal and short dexamethasone treatment (0.1–0.01 μg/g, days 1–4) on lung development in rats, focusing on its influence on peaks of cell proliferation around day 4 and of programmed cell death at days 19–21. By morphological criteria, we observed a dexamethasone-induced premature maturation of the septa ( day 4), followed by a transient septal immatureness and delayed alveolarization leading to complete rescue of the structural changes. The numbers of proliferating (anti-Ki67) and dying cells (TdT-mediated dUTP nick end labeling) were determined and compared with controls. In dexamethasone-treated animals, both the peak of cell proliferation and the peak of programmed cell death were reduced to baseline, whereas the expression of tissue transglutaminase (transglutaminase-C), another marker for postnatal lung maturation, was not significantly altered. We hypothesize that a short neonatal course of dexamethasone leads to severe but transient structural changes of the lung parenchyma and influences the balance between cell proliferation and cell death even in later stages of lung maturation.