Ammonia is an abundant fermentation product in the forestomachs of ruminants and the intestine of other species. Uptake as NH₃ or NH₄ ⁺ should modulate cytosolic pH (pH_i) and sodium-proton exchange via NHE. Transport rates of Na⁺, NH₄ ⁺ and NH₃ across the isolated rumen epithelium were studied at various luminal ammonia concentrations and pH values using the Ussing chamber method. The patch-clamp technique was used to identify an uptake route for NH₄ ⁺. The data show that luminal ammonia inhibits electroneutral Na transport at pH 7.4 and abolishes it at 30 mmol^.l^-1 (p<0.05). In contrast, at pH 6.4, ammonia stimulates Na transport (p<0.05). Flux data reveal that at pH 6.4, about 70% of ammonia is absorbed in the form of NH₄ ⁺, while at pH 7.4, uptake of NH₃ exceeds that of NH₄ ⁺ by a factor of ~4. The patch-clamp data show a quinidine sensitive permeability for NH₄ ⁺ and K⁺, but not Na⁺. Conductance was 135 ± 12 pS in symmetrical NH₄Cl solution (130 mmol^.l^-1). Permeability was modulated by the concentration of permeant ions, with P_K>P_NH4 at high and P_NH4>P_K at lower external concentrations. Joint application of both ions led to anomalous mole fraction effects. In conclusion, the luminal pH determines the predominant form of ammonia absorption from the rumen and the effect of ammonia on electroneutral Na transport. Protons that enter the cytosol through potassium channels in the form of NH₄ ⁺ stimulate and non-ionic diffusion of NH₃ blocks NHE, thus contributing to sodium transport and regulation of pH.