Fetal renal function in the sheep was investigated in a chronic preparation which permitted repeated evaluations of urine flow and osmolality as well as renal clearances in animals which were unanesthetized and remote from acute surgical stress. Measurements of fetal blood pressure, pH, osmolality, fetal growth in utero, and final outcome did not indicate an adverse effect of the experimental procedure on the fetus. Fetal urine flow and osmolality were highly variable during the early postoperative period. They did not stabilize until 3-6 days after surgery, when urine osmolality became markedly hypotonic (range 65-160 mOsm/kg H(2)O) and urine flow rose to approximately 0.14 ml/min.kg. Fluctuations in urine flow and osmolality in the early postoperative period were the result of tubular reabsorption of water rather than a change in the glomerular filtration rate. The inulin-(14)C clearance, used as a measure of the glomerular filtration rate, was 1.05 +/-0.05 ml/min.kg (mean +/-sem) for all animals studied. Urea, fructose, sodium, and chloride were partially reabsorbed by the fetal kidney, while creatinine was secreted. Continuous drainage of fetal urine for 18 days in one animal demonstrated that the fetus was able to excrete large amounts of water, sodium, and fructose without apparent detrimental effects.