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Techniques are described for insertion of vinyl catheters into the umbilical and limb vessels of the fetus of the sheep or the goat through small uterine incisions, with the ewes under spinal analgesia. The catheters are exteriorized and the fetus can be studied in its normal intrauterine environment. During constant infusion of antipyrine into a fetal limb vein, placental arteriovenous difference of antipyrine was measured, and fetal umbilical blood flow was calculated by the Fick method. "Carbonized" microspheres (50-µ diameter) labeled with various nuclides were injected into different venous sites in the fetus. The distribution pattern of the microspheres was used to determine the relative distribution of blood flow. Experimental evidence is provided that (1) there is no significant recirculation of microspheres, (2) the distribution of spheres is proportional to flow, and (3) circulatory physiology is not altered by injection of spheres. Quantitative data on the distribution of umbilical venous and superior and inferior vena caval return were obtained. It was possible to determine the actual blood flow to each of the fetal organs by relating the proportions of nuclide in each organ to that in the placenta. Total cardiac output was then calculable, taking into consideration the hemodynamic arrangement of the fetal circulation.