Left diaphragmatic hernias were produced surgically in ten control lambs at term, and in 19 lambs in 127-144 days' gestation. The hemodynamic and ventilatory changes that occur following delivery and attempted spontaneous breathing were noted. The herniated viscera effectively occluded the diaphragmatic defect, and all lambs were capable of developing negative intrathoracic pressure. Pulmonary artery pressure remained consistently elevated in all lambs with diaphragmatic hernia produced early in gestation. The lambs with diaphragmatic hernia early in gestation had hypoplastic lung with abnormally low air capacity and compliance.