Although water moves through the lipid domain of all biological membranes, there is strong evidence that facilitated or channel-mediated water transport exists in a number of cell plasma membranes and intracellular vesicles (l0, 37). The kidney proximal tubule contains water transporters in the cell apical and basolateral plasma membranes to facilitate rapid transcellular movement of water in response to small osmotic gradients. In the kidney collecting tubule and amphibian urinary bladder, transcellular water permeability is regulated by the vasopressin-induced movement of intracellular vesicles con­ taining functional transporters between an intracellular compartment and the plasma apical membrane (6, 14, 15,37). The regulation of water permeability in these tissues is essential for control of plasma volume and osmolality. The red blood cell has long been known to contain specialized water transporters (26, 32, 33); however, their physiological role is not clear. Although the evidence is less complete, specialized water transporting pathways have been proposed to exist in other cell types (11). Based on data in the kidney, red blood cell, and the artificial pore-forming agents gramicidin and amphotericin B, several characteristics have been associated with facilitated water transporters (10, 37). The osmotic water permeability coefficient (Pf) is high, generally greater than 0.01 cm/s. The ratio of osmotic-to-diffusional water permeability (P tiP d) is greater than unity,