We examined the components of net HCO3 reabsorption (H+/HCO3 secretion) in in vivo perfused distal tubules of anesthetized rats to determine the mechanisms by which dietary acid increases acidification in this nephron segment. Animals eating a minimum electrolyte diet drank either 80 mM NH4Cl or 40 mM (NH4)2SO4 for 7-10 days and were compared with controls drinking distilled H2O. Net HCO3 reabsorption in distal tubules perfused with HCO3 concentration ([HCO3]) similar to that in situ (5 mM) was higher in (NH4)2SO4 animals than in control (21.6 +/- 1.8 vs. 12.5 +/- 1.3 pmol.mm-1.min-1, respectively, P < 0.02), but that for NH4Cl (17.9 +/- 1.5 pmol.mm-1.min-1, P = 0.09 vs. control) animals was not. Calculated H+ secretion was not different among groups perfused with the 5 mM HCO3 solution, but calculated HCO3 secretion was lower in (NH4)2SO4 animals than control (-2.4 +/- 0.3 vs. -5.3 +/- 0.6 pmol.mm-1.min-1, respectively, P < 0.02), but that for NH4Cl (-7.2 +/- 0.7 pmol.mm-1.min-1, P = not significant vs. control) was not. When distal tubules were perfused with solutions containing higher [HCO3] (10 nM), both net HCO3 reabsorption and calculated H+ secretion were significantly higher than control in both acid-ingesting groups. The data show that reduced HCO3 secretion mediates the increased distal tubule acidification induced by dietary acid, particularly at the low tubule fluid [HCO3] in situ. The data also show that acid ingested as the Cl- compared with the SO4(-) salt does not reduce HCO3 secretion and less effectively increases acidification in this nephron segment.