Acid-base balance and amino acid metabolism are intimately related. Changes in acid-base balance influence the metabolic fate of many amino acids. Also, acid-base homeostasis is achieved in part by alteration of amino acid metabolism, not only in the kidney, but also in liver, muscle and splanchnic tissue. Glutamine is the primary amino acid involved in renal ammonia-genesis, a process intimately related to acid excretion. The metabolism of other amino acids, such a serine, glycine and the branched-chain amino acids, also appears to be influenced by acid-base balance. Conversely, the metabolic fate of various amino acids will influence the daily acid load experienced by the animal. Oxidation of amino acids contributes to the total acid and base load imposed on the pig. The basic (cationic) amino acids (lysine, arginine and histidine) yield neutral end-products plus a proton; sulfur (methionine and cysteine) amino acids are also acidogenic because they generate sulfuric acid when oxidized. The dicarboxylic (anionic) amino acids (aspartate and glutamate, but not asparagine and glutamine) consume acid when oxidized and thus reduce the acid load of the diet. Acid-base balance and related phenomena are discussed in the context of practical and metabolic aspects of amino acid nutrition.