Right and left ventricular outputs were measured by a dye dilution technique in fetal lambs before and during graded reduction of umbilical blood flow by constriction of the abdominal portion of the umbilical vein. PO2, PCO2and pH were measured on blood samples taken from the carotid artery and the ductus arteriosus. Oxygen saturation values were estimated from oxygen dissociation curves for fetal lambs' blood. With slight constriction of the umbilical vein, both ventricular outputs fell, but there was no significant difference between them for umbilical flows exceeding 150 ml/kg per min. With further reduction of umbilical flow, the output of the right ventricle fell more than that of the left. When flow was reduced to less than 120 ml/kg per min, right ventricular output was significantly higher than that of the left (P< 0.001). Oxygen saturation of blood from both the carotid artery and the ductus arteriosus fell with reduction of umbilical flow, but the difference between these values was not significantly altered. These observations show that major reduction of venous return may change the proportion in which various blood streams mix as they pass through the fetal heart.