The substrate and oxygen uptake by some organs in intact developing animals has been described, however, the kidney has not been studied. To examine substrate and oxygen uptake by the kidney, we implanted polyvinyl catheters into the renal vein, descending aorta, inferior vena cava, and urinary bladder of 11 fetal sheep (120–125 days gestation) and eight newborn lambs (1 day postnatal). Four days after surgery, blood samples were obtained simultaneously from the renal vein, aorta, and inferior vena cava for determination of oxygen content and saturation, and glucose and lactate concentrations. Renal blood flow was determined by the radionuclide-labeled microsphere method in the fetal lambs and by measuring,14C-inulin clearance in the newborn lambs. The fetal and newborn kidneys consumed oxygen at rates of 123 ± 16 and 785 ± 79 μmol/min/100 g kidney weight (mean ± SEM), respectively. The increase in oxygen consumption from the fetal to the newborn period was accompanied by an increase in oxygen extraction from 25–35%, a large increase in oxygen delivery from 418 ± 38 to 2231 ± 127 μmol/min/100 g, and marked increases in glomerular filtration rate and sodium reabsorprion (measured in six additional fetal sheep and the eight newborn lambs). This suggests that the postnatal increase in renal tubular activity is associated with an increase in oxygen consumption. Lactate was taken up by both fetal and newborn kidneys, and in nine of the 11 fetuses and in four of the eight newborns, there was net glucose release from the kidney.