These investigations are designed to study the physiologic maturation of the fetal lamb kidney and quantitate the contribution of the fetal kidney in compensating for various types of fetal distress. Specific investigations include: 1. Study of the relative rates of maturation of glomerular filtration and renal tubular function. (Glucose, bicarbonate sodium, and potassium reabsorption). 2. Studies of the effect of induced metabolic acidosis on the fetus and the contribution of the fetal kidney in compensating for fetal metabolic acidosis. 3. Studies of the responsiveness of the fetal kidney to vasopressin and fetal control of vasopressin secretion. 4. Investigation of the role of the immature kidney in the etiology of hyperphosphatemia and tetany of the newborn. These studies will be accomplished using a "chronic" intra-uterine sheep fetal-maternal preparation. Catheters will be placed in the fetal artery vein and the urachus, thus permitting serial studies to be performed utilizing blood and urine samples collected over the last half of gestation.