The inability to demonstrate adequate active transport in the ascending thin limb of the mammalian kidney has led to the development of models of concentration with only passive transport in the inner medullary loop of Henle. These models depend on very limited solute entry into the descending thin limb, an assumption counter to much of the experimental in vivo data. In addition, these models have not incorporated the vascular-tubular relationships in the renal medulla. We hypothesize that fluid enters the inner medulla in the descending thin limb slightly hyperosmotic to the fluid leaving the inner medulla in the interstitial-vasa recta compartment. We illustrate the hypothesis with a steady-state model of the medullary concentration process with no active transport in the inner medullary loop of Henle and with tubular permeability properties and solute concentrations consistent with experimental data. A difference of 33 mosmol/kg between descending thin limb and fluid leaving the inner medulla in the interstitial-vasa recta compartment results in an increase in formative urine osmolality from 530 mosmol/kg in the collecting duct at the outer-inner medullary boundary to 2,200 mosmol/kg in the ureteral urine. The hypothesis incorporates anatomical and physiological properties of the vascular and tubular structures in the outer medulla.