In fetal sheep, the effect of initiation of ventilation with an intact umbilical cord was studied by placing flow probes around the ductus arteriosus, carotid and pulmonary arteries and catheters in the carotid and pulmonary arteries for direct pressure measurements. With clamping of the umbilical cord prior to initiation of ventilation, carotid artery and pulmonary artery pressure increased, carotid artery blood flow increased initially and then decreased and pulmonary artery blood flow and right ventricular output decreased. These changes in carotid and pulmonary pressure and flow were accompanied by a decrease in heart rate. With just 30 seconds of ventilation prior to clamping of the umbilical cord, the fluctuations in heart rate, carotid and pulmonary artery pressure and flow were prevented and the cardiopulmonary transition was much more stable.