Cytochalasin B depresses the hydroosmotic response of the toad urinary bladder to vasopressin without affecting basal (bulk flow) permeability, diffusional permeability, or the hormone induced increase in short circuit current. Fine structural studies demonstrated that this macrolide fungal metabolite, in the presence of both an osmotic gradient and vasopressin, induces the formation of large intracellular vacuoles or 'lakes' in epitelial cells lining the bladder mucosa. Some surface changes (shortening and irregularity of microvilli, clumping of the glycocalyx, etc.) were reported by transmission electron microscopy. Scanning electron microscopy demonstrates that cytochalasin B drastically alters the mucosal surface morphology of the hormone stimulated bladder. Lesser changes were seen in the absence of vasopressin. In the presence of arginine vasopressin, excessive cellular swelling and possible rupturing, as well as surface membrane infolding and rippling, were seen in the cytochalasin treated tissues, The specific entity most affected by this treatment is the granular cell.