Cyclosporine A (CsA) causes distal renal tubular acidosis (dRTA) in humans and rodents. Because mice deficient in nitric-oxide (NO) synthase develop acidosis, we examined how NO production modulated H⁺ excretion during acid loading and CsA treatment in a rat model. Rats received CsA, l-arginine (l-Arg), or N^ω-nitro-l-arginine methyl ester (l-NAME), or combinations of CsA and l-NAME or l-Arg, followed by NH₄Cl (acute acid load). In vehicle-treated rats, NH₄Cl loading reduced serum and urine (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{HCO}_{3}^{-}\) \end{document}) and urine pH, which was associated with increases in serum [K⁺] and [Cl^–] and urine NH₃ excretion. Similar to CsA (7.5 mg/kg), l-NAME impaired H⁺ excretion of NH₄Cl-loaded animals. The combination CsA and l-NAME reduced H⁺ excretion to a larger extent than either drug alone. In contrast, administration of l-Arg ameliorated the effect of CsA on H⁺ excretion. Urine pH after NH₄Cl was 5.80 ± 0.09, 6.11 ± 0.13*, 6.37 ± 0.16*, and 5.77 ± 0.09 in the vehicle, CsA, CsA + l-NAME and CsA + l-Arg groups, respectively (*P < 0.05). The effect of CsA and alteration of NO synthesis were mediated at least in part by changes in bicarbonate absorption in perfused cortical collecting ducts. CsA or l-NAME reduced net \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{HCO}_{3}^{-}\) \end{document} absorption, and, when combined, completely inhibited it. CsA + l-Arg restored \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{HCO}_{3}^{-}\) \end{document} absorption to near control levels. Administration of CsA along with l-NAME reduced NO production to below levels observed with either drug alone. These results suggest that CsA causes dRTA by inhibiting H⁺ pumps in the distal nephron. Inhibition of NO synthesis may be one of the mechanisms underlying the CsA effect.