The maturation of renal function was investigated by clearance studies in 11 fetal, 9 young, and 4 adult guinea pigs undergoing salt loading (NaCl 1.5%, 75 mul min-1 100 g-1) and in 10 young and 4 adult animals under nondiuretic conditions. The fetal preparation described makes it possible to obtain two to four steady clearance periods for each animal. In the fetus, GFR increased significantly with age, but less than in the newborn. Fractional water and sodium reabsorption was generally low, and net renal secretion of potassium was observed in some cases. Fetal urine was sometimes slightly hypertonic. The effectiveness of the diuretic and natriuretic response following saline expansion was smaller in young than adult animals. The fractional excretion rate of solutes and water increased as a function of age in the young animal and the urinary Na/K concentration ratio showed a marked drop around birth. This latter observation suggests that during the first days of life the effect of saline expansion on the proximal tubule is partially blunted by enhanced sodium reabsorption from the distal nephron due to the high concentration of circulating aldosterone.