Newborn rabbits maintain a state of hypochloremic metabolic alkalosis with plasma HCO3- concentrations generally exceeding 27 mM. The large amounts of potential net base in mother's milk probably contribute to the generation of this alkalosis. We surmised that immature handling of H+ or HCO3- by the neonatal collecting duct helps maintain this alkalosis. Net HCO3- transport was measured in perfused collecting ducts taken from maturing rabbits, in solutions simulating adult rabbit plasma ultrafiltrate and then in presence of Cl(-)-free bathing solution. Cortical collecting ducts (CCD) from newborn and 4-wk-old rabbits failed to secrete HCO3- under baseline conditions and could not be stimulated to secrete HCO3- in Cl(-)-free bath. Neonatal segments perfused at very slow flow rates showed significant HCO3- absorption; inhibition of HCO3- secretion by removal of luminal Cl- revealed a substantial HCO3- absorptive flux. Segments from 6-wk-old and mature animals secreted net HCO3- and generally showed more than a fivefold increase in HCO3- secretion after Cl- removal from the bath. Outer medullary collecting ducts (OMCD) from newborn rabbits absorbed HCO3- at rates approaching that of mature segments. We conclude that relatively high rates of HCO3- absorption in OMCD and lack of HCO3- secretion in CCD may contribute to the metabolic alkalosis of the neonatal rabbit.