During antidiuresis renal medullary cells were previously found to accumulate large amounts of organic osmolytes, namely sorbitol, myo-inositol, glycerophosphorylcholine (GPC), and betaine. Large quantities of amino acids are also present in the renal medulla, but it has been questionable whether the levels of medullary amino acids are osmotically regulated. Therefore we directly measured 26 different amino acids, as well as sorbitol, myo-inositol, GPC, and betaine alone the corticomedullary axis of rats that were either salt loaded, after infusion of hypertonic NaCl solution, or were hydrated, after infusion of hypotonic NaCl solution. The amounts of sorbitol, myo-inositol, GPC, and betaine are greater in the inner medullas of salt-loaded rats compared with hydrated rats. In addition, the amount of taurine is much greater in the inner medullas of salt-loaded rats. Aspartic acid also increases in salt-loaded rats but to a lesser extent. There are substantial gradients of taurine, aspartic acid, and some of 24 other measured amino acids along the corticomedullary axis. However, taurine and aspartic acid are the only measured amino acids that increase significantly during salt loading.