The body regulates acid and base within a definite and very precise range; any excess of either will cause disturbances in enzymatic reaction, body maturation and growth. The kidneys play a prominent role in the excretion of acid. Recently developed methods permitting the quantitative measurement of endogenous acid production from food metabolism, skeletal growth, etc. are evaluated. The clinical application of such principles to the study of the acidogenic properties of milk formulae, amino acid infusates etc. are also reviewed, together with comments on the limitations of such balance methods.