Although the mammalian urinary tract (UT) is generally held to be soley a transit and storage vehicle for urine made by the kidney, in-vivo data suggests reabsorption of urea and other urine constituents across UT epithelia. To determine if UT tissue concentrations are increased as a result of such reabsorption we measured urea nitrogen and creatinine concentrations and examined for urea transporter B presence in bladder, ureter and other tissues obtained from rats and dogs. Mean urea nitrogen and creatinine concentrations in UT tissues of rats and dogs were 3-7 fold higher than those in serum and comparable to those in renal cortex. In rats subjected to water restriction or water loading urea nitrogen concentrations in bladder tissues fell inversely with the state of hydration, were proportional to urine urea nitrogen concentrations, and were greater than the corresponding serum urea nitrogen concentration in every animal. Immunoblots of rat and dog UT tissues demonstrated the presence of urea transporter-B in homogenates of bladder and ureter and immunocytochemical analysis localized the urea transporter to epithelial cell membranes. These findings are consistent with the notion that urea and creatinine are continuously reabsorbed from the urine across the urothelium, urea in part via the urea transporter-B , and that urine is thus altered in its passage through the urinary tract. Urea reabsorption across UT epithelia may be important during conditions requiring nitrogen conservation and may contribute to pathophysiologic states characterized by high blood UN such as pre-renal azotemia and obstructive uropathy.