Preterm infants are at high risk of developing ventilator-induced lung injury (VILI). We have used an animal model of in utero ventilation (IUV) to investigate the separate effects of ventilation and acute oxygen exposure on the very immature lung. Fetal sheep were ventilated in utero at 110d gestation for 6h with either 100 , 21 or 0 (100 nitrogen) oxygen (n=5 each) and survived in utero, without further ventilation, until tissue collection at 118d. Non-ventilated 110d and 118d fetuses were used as controls. All IUV exposed fetuses had reduced secondary septal crest densities and increased elastin staining irrespective of the inspired oxygen concentration. IUV with 100 and 21 oxygen, but not 100 nitrogen, increased lung tissue volumes and myofibroblast differentiation and apoptosis within the distal lung parenchyma in a dose dependent manner. This study shows that IUV without oxygen can reduce alveolarization, whereas ventilation with oxygen (6h), even at levels found in air (21 ), increases distal lung tissue volumes, elastin deposition, myofibroblast differentiation, and apoptosis.