The urinary bladder of Bufo marinus excretes H+ and NH+4, and the H+ excretion is increased when the animal is placed in metabolic acidosis. The mitochondria-rich (MR) cells mediate the H+ excretion by the bladder. The purpose of this study was to determine if there is a change in MR cells of the bladder during metabolic acidosis. Bladders from normal toads and from toads that had been placed in metabolic acidosis were used. The bladders were mounted between plastic chambers and H+ excretion measured. The bladder was then fixed and prepared for scanning (SEM) and transmission (TEM) electron micrograph studies. SEM's at low magnification were used to count the various cell types and the TEM's were used to confirm the different cell types. Fields were randomly selected and a total of 2500 cells counted in each group. The bladders from toads in metabolic acidosis had a consistently higher ratio of MR cells to granular cell than did the normal bladders. These results indicate that during metabolic acidosis there is an increased number of MR cells in the bladder, and this increased the bladder's capacity to excrete H+.