The presence of vascular shunts in the fetal circulation results in mixture of oxygenated blood returning from the placenta with venous blood returning from the fetal body. In fetal sheep with chronically implanted vascular catheters we found during normal oxygenation that 22% of umbilical venous blood recirculated to the placenta without being presented to the fetal tissues (left-to-right shunt analogue) while about half of systemic venous blood recirculated to the fetal body and not to the placenta for oxygenation (right-to-left shunt analogue). These shunts comprised 34% of cardiac output and increase the workload on the fetal heart, as they are ineffective in terms of oxygen uptake and delivery. In normally-oxygenated fetuses, 11% of cardiac output was comprised of umbilical venous blood recirculating to the placenta (left-to-right shunt analogue) whereas 23% of cardiac output consisted of systemic venous blood circulating to the fetal body (right-to-left shunt analogue). During induced maternal-fetal hypoxia (10% O2 to ewe), although the percentage of ineffective cardiac output did not change (31%), the left-to-right shunt analogue increased to 19% and the right-to-left shunt analogues decreased to 12% of cardiac output. Hypoxia also resulted in an increase in the percentage of umbilical venous blood which bypassed the liver through the ductus venosus (control 57 +/- 12%; hypoxia 65 +/- 12%, P < 0.02), causing the proportion of umbilical venous blood contributing to the fetal cardiac output to increase from 27 +/- 10% during normoxia to 39 +/- 9% during hypoxia (P < 0.005). This factor, together with the redistribution of cardiac output during hypoxia, resulted in a 100% increase in oxygen delivery from umbilical venous blood to the myocardium and a maintenance of the umbilical vein derived oxygen supplied to the brain and placenta, despite a 40% reduction in total available oxygen in umbilical venous blood.