Prolonged hyperoxic exposure is associated with impaired alveolarization of the lung in both the rat and the human neonate. Elastin is currently thought to play a pivotal role in the alveolarization of the lung by providing the structural framework around which new alveoli will develop. Previous studies in both the rat and the human neonate have demonstrated a risk for proteolytic destruction of lung elastin associated with prolonged hyperoxic exposure. The present study was undertaken to determine whether continuous exposure to 100% oxygen during the period of alveolar development in the rat (Days 4 to 13) would alter lung elastin. Parenchymal lung elastic fiber length, volume density of parenchyma, mean linear intercept, and internal surface area were quantitated using morphometric techniques, and the values were compared in control, oxygen-exposed, and malnourished rat pups. Stereologic measurements indicated that total elastic fiber length was significantly greater in lungs of control pups than in lungs of either the oxygen-exposed or the malnourished pups. Examination of sections of lung tissue 20 to 30 microns thick indicated altered elastic fiber structure and numerous alveolar fenestrae only in the hyperoxic pups. The results of these studies demonstrated that hyperoxic exposure during alveolarization alters both total length and structure of lung elastic fibers and suggest that impaired lung development might be due in part to these observed changes.