T he mammalian antidiuretic hormone, 8-arginine-vasopressin, was found to increase net mucosal-to-serosal urea flux across the isolated toad urinary bladder 13-fold. This urea flux was accompanied by a 24-fold increase in solute-linked water movement across the membrane. Net urea flux and urea-linked volume flux were inhibited by 50% or more when thiourea was added to the mucosal medium at concentrations equal to those of urea. In contrast, thiourea did not inhibit osmotic water flux across the bladder in the presence of vasopressin. These observations are consistant with the view that thiourea and urea compete for a common site on a membrane carrier molecule.When bladders were exposed to vasopressin on the serosa and subsequently fixed with 1% glutaraldehyde on the mucosa, they were found to retain 74% of their prefixation permeability to urea. Net urea flux across these fixed bladders (in the absence of vasopressin) was markedly inhibited by thiourea, whereas osmotic water flux was not inhibited. These studies suggest that vasopressin induces the formation of “urea-channels” in the membrane that can be preserved by glutaraldehyde and blocked by thiourea.