The toad urinary bladder is a well-characterized system for the study of epithelial transport. Our analysis of the relationship between luminal membrane particle aggregates seen by freeze-fracture electron microscopy (which we use as an index luminal membrane osmotic water permeability) and whole tissue water permeability strongly suggests that osmotic water flow across the bladder is limited by two barriers arranged in series: a) the granular cell luminal membrane whose permeability increases greatly when the bladder is treated with the antidiuretic hormone vasopressin (ADH), and b) a fixed resistance barrier which contributes significantly to the total osmotic flow resistance across the bladder when the luminal membrane is made permeable by vasopressin. We will use the relationship between aggregate frequency and tissue water flow to measure these two barriers to water flow independently, and to characterize their alteration by vasopressin, temperature, glutaraldehyde fixation, water flow and tissue stretch. By measuring the epithelial cell osmolality during trans-bladder water flow we will localize osmotic gradients across the tissue, and thereby the sites of significant flow resistance. By measuring the pattern the isotope efflux from prelabeled tissues, we will be able to define the barriers to diffusion of water and nonelectrolyte solutes and accurately measure the effect of vasopressin on the luminal membrane's permeability to these substances.