We examined whether dietary acid that increases net acid excretion (NAE) without measurably decreasing plasma pH or total CO₂ (tCO₂) causes net acid retention. Control rats drinking distilled H₂O were compared with those drinking 40 mM (NH₄)₂SO₄, 40 mM Na₂SO₄, or drinking Na₂SO₄ and given aldosterone (Na₂SO₄ + Aldo) to increase NAE without dietary acid. Systemic plasma tCO₂ increased in Na₂SO₄ + Aldo animals, but systemic and stellate vessel plasma tCO₂ and pH were not different from control among remaining groups. NAE increased in (NH₄)₂SO₄ and Na₂SO₄ + Aldo but not in Na₂SO₄ animals. Blood base excess (BBE) decreased compared with its respective baseline in (NH₄)₂SO₄ (0.44 ± 0.06 vs. 0.66 ± 0.04 µmol/ml; P < 0.01, paired t-test), increased in Na₂SO₄ + Aldo (0.79 ± 0.05 vs. 0.61 ± 0.03 µmol/ml; P < 0.04, paired t-test), but was unchanged in Na₂SO₄ animals. Renal cortical H⁺ content assessed by microdialysis of the renal cortex in situ increased in (NH₄)₂SO₄, decreased in Na₂SO₄ + Aldo, but was unchanged in Na₂SO₄ animals. The data show that dietary acid sufficient to increase NAE without decreasing plasma tCO₂ or pH nevertheless decreases BBE and increases renal cortical acid content, consistent with net acid retention.