Three to five measurements of fetal cardiac output and its distribution were made using radiolabeled microspheres in each of 12 sheep fetuses in whom fetal plasma insulin concentration was varied by exogenous infusion of insulin to the fetus. Blood concentrations of oxygen and glucose as well as blood gases, pH, hematocrit, and plasma insulin concentrations were also measured. Both fetal arterial oxygen content and whole blood glucose concentration fell as fetal insulin concentration rose. Fetal cardiac output rose as insulin concentration increased and was accompanied by an increase in fetal heart rate. Increased insulin concentration was associated with increased flow to the heart, stomach, placenta, and carcass while blood flow to the fetal brain, lungs, liver, kidneys, spleen, and intestines was not affected. As plasma insulin concentration rose, the percentage of the cardiac output distributed to the fetal heart and upper carcass increased; that distributed to the fetal brain, lungs, liver, stomach, intestines, and lower carcass remained unchanged; while that distributed to the kidneys, spleen, and placenta decreased. When expressed as a fraction of cardiac output, an additional 7% (from 39 to 46%) of the cardiac output was distributed to carcass over the range of insulin concentrations studied. Most of the increased portion of the fetal cardiac output distributed to the fetal carcass during hyperinsulinemia was accounted for by a decrease in the percentage of fetal cardiac output perfusing the placenta.