Hyaluronan is an important component of the lung extracellular matrix, with a high capacity for water immobilization, but information on perinatal changes in the lung hyaluronan concentration and their association with changes in the lung water content is limited. In this study, conducted both in rabbit pups and in human infants, we investigated changes in the hyaluronan concentration and distribution in the lung and in the lung water content after preterm or term birth, and changes produced by common antenatal and postnatal pathological conditions and treatments. In rabbit pups, we found a gradual decrease in lung hyaluronan concentration and in the intensity of alveolar hyaluronan staining with advancing gestational age at birth in late gestation, but no further changes during the first 7-9 days of life. The lung water content was uniformly high before birth, but decreased significantly after preterm delivery or at birth at term. Postnatal exposure of newborn preterm or term rabbit pups to hyperoxia for 4-9 days resulted in an increase in both lung hyaluronan concentration and lung water content. This was accompanied by more intense hyaluronan staining, mainly in the alveolar walls. Antenatal exposure of rabbit pups to betamethasone or terbutaline resulted in a lower lung hyaluronan concentration at preterm birth, associated with less intense hyaluronan staining in alveolar walls, without altering the lung water content. Betamethasone exposure had a maximal effect at 25 days of gestation (term = 31 days), decreasing thereafter with advancing gestation, while terbutaline exposure resulted in a gradually increasing effect during late gestation, with a maximum at 29 days. In deceased infants born at a gestational age of < 32 weeks, the lung hyaluronan concentration at death was most strongly associated with the gestational age at birth. It also covaried with sex, antenatal steroid administration, intrauterine bleeding, mode of delivery, birth weight, IRDS, and surfactant treatment. In infants born at a gestational age of > 33 weeks there was a weaker association between lung hyaluronan concentration and gestational age. In this group, the lung hyaluronan concentration was associated with administration of a high concentration of oxygen, and covaried with maximal ventilatory pressure, and lung water content.