Antidiuretic hormone (ADH) dramatically increases the water permeability of toad urinary bladder by insertion of unique highly selective water channels into the apical membranes of granular cells. Before ADH stimulation, water channels are stored in high concentrations in the limiting membranes of large cytoplasmic vesicles called aggrephores. ADH stimulation causes aggrephore fusion with the granular cell apical membrane and increases water permeability. Transepithelial osmotic water flow causes a rapid attenuation of the ADH-elicited increase in water permeability through a process called flux inhibition. Flux inhibition is due to retrieval of ADH water channels by apical membrane endocytosis. When phosphoproteins of intact bladders are labeled with (32P)orthophosphate, the 32P content of 34-, 28-, and 17-kDa proteins is increased by ADH stimulation. When flux inhibition occurs, the 32P-labelling of a 15.5-kDa protein is reduced to approximately one half its original value (Konieczkowski M, Rudolph SA, J Pharmacol Exp Ther 1985;234:515). These observations have been confirmed, and these studies have been extended, by using a combination of subcellular fractionation and membrane protein chemistry techniques. All four of these phosphoproteins are present in membrane fractions of granular cells. Analysis of membrane proteins by a combination of Triton X-114 partitioning and an alkaline stripping technique reveals that the 28- and 17-kDa species are integral membrane proteins of unknown function. In contrast, the 32P-labeled 15.5-kDa protein is a peripheral membrane protein. It is attached to the cytoplasmic (outer) surface of highly water-permeable vesicles retrieved during flux inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)