Histochemical changes were observed in the epithelium of the toad urinary bladder following in vitro and in vivo stimulation by aldosterone to determine the cell types affected by this hormone. A marked pyrinophilia was observed in comparison to control tissue affecting all cell types, as well as slight increases in cytoplasmic PAS staining within the granular cells, under both sets of conditions. An increased number of mitochondria was demonstrated with the PTAH stain within the granular cells under in vivo conditions but not in vitro. Under in vivo conditions, marked increases in the enzymes of the oxidative pathway (DPND, TPND, SDH, and cytochrome oxidase) were observed in granular cells. Enzymes of the oxidative pathway were unchanged under in vitro conditions was more intensely localized to the cell membrane and cytoplasm of all cells. Granular cells from in vitro studied tissue were slightly more reactive for G6PD and 6PGD than control tissue; granular cells were even more heavily reactive for these enzymes under in vivo conditions. Thus, under prolonged aldosterone stimulation, the granular cell approached the histochemical profile of the mitochondria-rich cell. The hormone may therefore act upon the granular cell, which comprises the largest number of cells within the epithelium, as well as the mitochondria-rich cell.