The present in vitro microperfusion study examined apical membrane Na+/H+ antiporter and basolateral membrane Na(HCO3)3 symporter activity in newborn and adult juxtamedullary proximal convoluted tubules. Proton fluxes were determined from the initial rate of change of intracellular pH after a change in the luminal or bathing solution, buffer capacity, and tubular volume of newborn and adult tubules. Intracellular pH (pHi) was measured fluorometrically using the pH-sensitive dye (2',7')-bis (carboxyethyl)-(5,6)-carboxyfluorescein (BCECF). Apical Na+/H+ antiporter proton flux, assayed by the effect of sodium removal (147----0 meq/liter) on pHi, was one-third the adult level for the first 2 wk and doubled in the 3rd wk of life. Adult levels were achieved by 6 wk of age. Na+/H+ antiporter activity was not detected on the basolateral membrane of 1-wk-old newborns, indicating that polarity of this transporter was already present. Basolateral membrane Na(HCO3)3 proton flux, assayed by the effect of a bath bicarbonate change (25----5 meq/liter) and by a bath sodium change (147----0 meq/liter) on pHi, was 50-60% of adult values in 1-wk-old newborns. Basolateral membrane Na(HCO3)3 proton flux assayed by a bath bicarbonate change (25----5 meq/liter) remained at 50-60% of adult values for the 1st mo of life and increased to adult levels by 6 wk of age. This transporter not only plays a role in net acidification, but is an important determinant of cell pH in newborn juxtamedullary proximal convoluted tubules.