Abstract It is agreed that acids and bases may be defined in terms of transfers of hydrogen ion. In the body, however, since hydrogen ion changes are almost completely buffered, they cannot be measured adequately as such but only in terms of the accompanying reciprocal change in buffer anions. These are conveniently termed "buffer base" and as proton acceptors may indeed be defined as "base." The principle of electroneutrality permits this same numerical quantity to be derived as the excess of cations over fixed anions and provides another, and often more convenient, way of determining the same quantity. Changes in each of the factors involved occur simultaneously and are different ways of looking at the same thing. It is not meaningful to try to call any one the "cause" of another. Since the same quantity is determinable as either cation or anion it is most convenient to simply call it buffer "base." Change in buffer base quantitates a non-respiratory or "metabolic" alteration in acid-base balance. In pure "respiratory" disturbances buffer base does not change, prior to secondary responses. Here, hydrogen ions are redistributed among the buffer anions, i.e., there are reciprocal alterations in the bicarbonate and non-bicarbonate buffer anions. Thus determination of buffer base separates non-respiratory or metabolic and respiratory acid-base disturbances. Individual ionic effects may be conveniently examined with respect to their effect on buffer base. Although hydrogen ion is the reference point for the entire development (which is consistent with the Bronsted theory), it is convenient to talk directly about effects of other ions rather than to return to the hydrogen ion at every step. We believe it is easier and clearer, once the idea is explained, to say that "an increase of fixed cation over fixed anion is an increase in buffer base" than to say that "an increase in fixed cation over fixed anion is a convenient way to determine an increase in total buffer anions, which in turn is a convenient way to determine a decrease in hydrogen ion." Either is quite acceptable and neither statement leaves the erroneous impression that it is possible to describe a complex situation with a single term or that the pH or hydrogen ion concentration alone adequately describes the disturbance.