Changes in epithelial cell morphology, especially at the apical plasma membrane, are frequently cited as initial evidence for antidiuretic hormone (ADH)-induced increase in membrane permeability. The effects of ADH and agents that alter and modify calcium and prostaglandin concentrations on the morphology and cytology of the epithelial cells of frog (Rana pipiens) urinary bladder are presented using the techniques of transmission and scanning electron microscopy. It was found that, like ADH, calcium ionophore, A23187, produce intense microvilli formation, microfilament mobilization and an increase in the density of granules and membrane associated vesicles, suggesting a prominent role of calcium in these processes. Moreover, our results suggest that these membrane and cytosolic transformations may be mediated in part through prostaglandin formation, as exogenous PGE2 mimicked these effects, and indomethacin, a prostaglandin synthesis inhibitor, attenuated ionophore's effect on luminal cytomorphology. However, unlike ADH, prostaglandins and ionophore inhibit hormonal-induced increase in transepithelial water flow. These results suggest that other components more distal to the luminal membrane, perhaps the basolateral membrane, may be rate-limiting for transepithelial water flow and possibly are regulated by either changes in calcium concentrations or prostaglandins.