Whether altered reabsorption of bicarbonate by the superficial distal nephron contributes to the renal response to a chronic dietary bicarbonate challenge is not known. These micropuncture studies were done in control, NaHCO3-loaded, and NaCl-loaded Munich-Wistar rats to determine whether this dietary maneuver altered acidification in this nephron segment as evaluated in situ. Urinary bicarbonate excretion of NaHCO3-loaded animals was higher than control for each day of a 7-day study period, but bicarbonate excretion for animals identically loaded with NaCl was not sustained above control. Bicarbonate transport by the superficial distal nephron was assessed using paired collections from this segment. Net bicarbonate reabsorption was observed in all three groups. Reabsorption was lower in NaHCO3-loaded vs. control (11 vs. 25 pmol/min, P less than 0.01) but that for NaCl-loaded animals was not. The data indicate that reduced reabsorption of bicarbonate by the superficial distal nephron of rats contributes to the renal response to a chronic dietary bicarbonate challenge as evaluated in situ.