The premature newborn's response to increasing degrees of Lt→Rt PDA shunt is unknown. In 14 fetal lambs at 128d gestation (term 147d), we infiltrated the ductus with formalin and placed a mechanical occluder about it so that its patency could be regulated. 3d later the lamb was delivered, given sheep surfactant, paralyzed and mechanically ventilated. Microsphere measurements of left ventricular output (LVO) and its distribution were made prior to and 1.25 h after a change in ductus patency. Twenty four changes from open→closed or closed→open were assigned randomly from 3 to 7 h after delivery. Lambs with ductus shunts < 50% LVO (38%) had a significant increase in LVO (167 vs. 268 ml/kg/min:closed vs. open) with no change in effective systemic blood flow or BP. There was a significant increase in coronary flow but no change in flow to the kidneys, GI tract, brain, or carcass (ml/100 gm/min). Although lambs with shunts > 50% LVO (65%) had a significant increase in LVO (164 vs. 300 ml/kg/min), there was a significant decrease in effective systemic blood flow and BP. There were significant decreases in flow to the kidneys, GI tract, brain and carcass. Coronary flow was unchanged but there was a decrease in subendocardial flow. There was no change in HR or pulmonary compliance at either degree of shunt. Preterm lambs are capable of compensating for moderate degrees of PDA shunt by increasing preload and increasing stroke volume; highe degrees of shunt significantly decrease organ blood flow.