The calcium antagonist verapamil and the calcium ionophore A23187 have been shown to inhibit the hydro-osmotic actions of antidiuretic hormone (ADH) presumably by different mechanisms. Presently, urinary bladders of the frog (Rana pipiens) were examined under SEM following exposure to calcium ionophore A23187, verapamil and ADH in the presence and absence of an osmotic gradient. Cells exposed to ADH show marked changes in surface substructure which is accompanied by an expansion of microridges, cell borders and the appearance of microvilli in the granular cells. The microvilli are pronounced and appear at the junction of microridges. In the presence of an osmotic gradient, ADH induces granular cell swelling and some cells show a blistering effect. Calcium ionophore, in the absence of an osmotic gradient, induced pronounced morphological changes in the granular cells, where the microvilli become prominently visible as 'finger-like' projections. This effect may be due to the action of calcium in promoting elongation of microtubules. Cells exposed to ionophore plus ADH are indistinguishable from ionophore alone. The most apparent effect of verapamil on surface substructure was on the elevation of the mitochondrial-rich cells above the surrounding granular cells. These cells show some degree of separation from the granular cells and are accentuated in tissues exposed to verapamil plus ADH. The present observations suggest that these agents, verapamil and calcium ionophore, have marked effects on cellular morphology. These actions are mediated through alterations in calcium movements and reflect the relative importance of cellular calcium in transepithelial water flow and the actions of antidiuretic hormone.