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Bicarbonate reabsorption is classically regarded as a rate-limited process characterized by saturation kinetics. The tubular maximum (Tm), however, varies with glomerular filtration rate. Thus bicarbonate reabsorption, in common with sodium reabsorption, is characterized by glomerulo-tubular balance. The examination of bicarbonate reabsorption is accomplished using the bicarbonate titration technique; however, this method in its traditional form leads to marked expansion of extracellular fluid (ECF) volume. The possibility exists, therefore, that glomerulo-tubular balance for bicarbonate is altered by the volume expansion and thus that the classic pattern of reabsorption may actually reflect inhibited bicarbonate reabsorptive capacity. The present studies were performed in rats to examine this possibility. Bicarbonate titration studies were performed in two groups of animals: (a) those in which ECF volume expansion was minimized; and (b) those in which ECF volume expansion was exaggerated. In the first group, no Tm for bicarbonate was observed either in the majority of individual rats studied or in a group plot for all rats studied despite the fact that plasma bicarbonate concentrations were increased to values in excess of 60 mEq/liter. In the second group, a clear Tm was demonstrated both in individual animals and in group data and there was a lowered threshold for the excretion of bicarbonate. The data thus lend support to the view that the "normal" Tm for bicarbonate may actually represent an inhibited level of bicarbonate reabsorption induced by ECF volume expansion.