Congential diaphragmatic hernia of the Bochdalek type occurs in 1:1,200 human stillbirths, and approximately 1:5,000 human live births. The majority of the still-births have a major, fatal congenital anomalies. At least half of the babies who are born alive with diaphragmatic hernia, die before surgery can be performed. Of those surviving to reach a surgical center, a further 30 to 50% die. The overall mortality for this lesion, then, approaches 70%. The recent literature stresses that although more and more babies are being operated upon at an earlier stage in life, the mortality for this lesion has not improved at all. One suggestion for improving survival has been to attempt to develop the approach of fetal surgery. For this to be successful, an experimental model needs to be developed. There is an extensive literature on the creation of diaphragmatic hernia in the fetal lamb and it is proposed to develop this model. Diaphragmatic hernia will be created in fetal lambs through a left thoracotomy at 60 to 90 days gestation. These will be repaired between 120 and 130 days gestation. Aspects to be studied will be: A) The earliest time at which it is feasible to creaste a diaphragmatic hernia in the fetal lamb. B) Whether it is possible to achieve survival of a lamb in which a diaphragmatic hernia has been created and repaired in utero, after the lamb is delivered. C) If lamb survival is achieved, then attention will be directed to the detailed morphologic changes in the lungs following both the creation and the repair of the diaphragmatic hernia. D) The latest time in gestation at which the repair needs to be carried out in order to achieve survival of the lamb.