Rat urinary bladder epithelium consists of basal, intermediate and superficial cell layers. The superficial cells (SP cells) were desquamated after 12-O-tetradecanoylphorbol-13-acetate (TPA)-treatment, and the morphological changes of the regenerating epithelium were investigated by scanning and transmission electron microscopy. Twenty-four hours after TPA-treatment, the epithelium had principally two cell layers and no typical SP cells were observed. During the next 24 hours, the epithelial cell layers became irregular as the epithelial cells with multiform cell projections interdigitated with each other and more than five layers of cell stratification were observed. These cells were bright and had large round nuclei. Their luminal surfaces at this stage were triangular or rectangular. Four days after TPA treatment, epithelial stratification was reduced to approximately three cell layers and some of the luminal cells had recovered the fusiform vesicles. Eight days after TPA treatment, most regions of the epithelium had returned to normal and typical SP cells with many fusiform vesicles had regenerated. These results indicate that the regeneration of bladder epithelium after TPA treatment occurred within about one week.