There are clinically important differences between the neonatal and adult kidney. The neonatal kidney has a lower glomerular filtration rate and immature tubules compared to the adult. Tubular immaturity predisposes neonates to develop fluid and electrolyte disorders. The adult proximal tubule reabsorbs 80 percent of the filtered bicarbonate and 60 percent of the filtered NaCl. Most of proximal tubule acidification in the mature proximal tubule is due to the apical Na+/H+ exchanger (NHE3). The Na+/H+ exchanger in parallel with a Cl-/base exchanger also mediates transcellular NaCl transport. Half of NaCl transport in the mature segment is passive and paracellular. Neonatal proximal tubules have almost an undetectable level of NHE3. This proposal will examine what produces the developmental increase in NHE3 and in proximal tubule NaCl transport. There is a 100-fold increase in corticosterone level and a 10-fold increase in thyroid hormone level during the first three weeks of life in the rat. We have recently demonstrated that this rise in glucocorticoids mediates most, but not all, of the increase in NHE3. The first aim will examine the mechanisms whereby glucocorticoids increase NHE3. The second aim will examine if the maturation of NHE3 is totally prevented in a novel animal model, a hypothyroid glucocorticoid deficient neonatal rat. In addition to the developmental changes in active transport, we have recently demonstrated that there are significant differences in the paracellular pathway in immature tubules, which impact the passive component of NaCl transport. Whereas one half of NaCl transport is paracellular in the adult proximal tubule, there is essentially no passive NaCl transport in the neonatal segment. The third aim will characterize the physiologic and molecular characteristics of the neonatal and adult proximal tubule paracellular pathway to determine why there is no passive NaCl transport in the neonatal proximal tubule.