Transitional epithelium of mammalian urinary bladder is a functional barrier which prevents passage of urine into the submucosa. In a previous study, we found uptake by the epithelial cells of horseradish peroxidase (HRP) instilled into the mouse bladder lumen. In the present study, the ability of bladder epithelial cells to take up HRP, which was visualized by the diaminobenzidine (DAB) reaction, was compared among several mammalian species by light and electron microscopy. In mouse, rabbit and dog bladder, the superficial cells of the transitional epithelium showed positive reactions for HRP, while cells in lower epithelial layers were DAB-negative. Some of the reaction products could be seen as a number of granules not surrounded by a membrane. The intercellular space of the epithelium was DAB-negative. In rat and guinea pig bladder, however, all urothelial cells showed negative reactions for HRP indicating that no uptake of HRP had been occurred in these animals. These results indicate that the uptake of HRP by the superficial cells of the bladder epithelium differs among mammalian species, indicating some functional differences in their superficial cells.