Development of urea transport inhibitors as clinically useful diuretics requires precise understanding of urea transepithelial transport in all nephron segments. Urea and Na+ transepithelial permeabilities in rat inner medullary thin limbs of Henle's loops are very high; however, pathways for urea and Na+ fluxes are unknown. Inner medullary thin limbs include the upper descending thin limb (upper DTL), which is AQP1-positive, lower descending thin limb (lower DTL), which is AQP1-negative and ascending thin limb (ATL). When measured at 37° C and 16° C, urea permeabilities of isolated perfused upper DTLs are unchanged (~50 E-5 cm/s); however, at 16° C urea permeabilities in lower DTLs are significantly and reversibly reduced from their values at 37° C by about 33% (from ~420 E-5 cm/s to about 280 E-5 cm/s). Qualitatively identical temperature effects are seen for transepithelial Na+ permeabilities in thin limb segments. At least two transepithelial pathways may account for urea and Na+ fluxes in thin limbs of Henle's loops – one temperature dependent and one temperature independent. Pathway differences may depend on a combination of carrier-mediated transport and/or gating of channels.