Preterm infants are at high risk of developing ventilator-induced lung injury (VILI), which contributes to bronchopulmonary dysplasia. To investigate causes of VILI, we have developed an animal model of in utero ventilation (IUV). Our aim was to characterize the effects of IUV on the very immature lung, in the absence of nonventilatory factors that could contribute to lung pathology. Fetal sheep were ventilated in utero at 110 d gestation for 1, 6, or 12 h (two groups; n = 5 each). Lung tissue was collected at 12 h after initiating IUV in the 1, 6, and one 12 h IUV groups. Lung liquid was replaced in the second 12 h IUV group and tissues collected at 117 d. Operated, nonventilated 110 and 117 d fetuses were controls. IUV reduced secondary septal crest densities, simplified distal airsacs, caused abnormal collagen and elastin deposition, and stimulated myofibroblast differentiation and cellular proliferation. IUV causes VILI in very immature lungs in the absence of other complicating factors and reproduces bronchopulmonary dysplasia -like changes in lung morphology. IUV offers a novel method for dissociating VILI from other iatrogenic factors that could contribute to altered lung development caused by VILI.