Lysine is a major constituent of amino acid parenteral nutrition solutions which have recently been shown to increase the severity of various types of acute renal failure in the rat. In previous studies the authors have shown that high-dose lysine alone is capable of causing acute renal failure. However, it has remained unclear what the morphologic expression of this type of acute renal failure is in the maintenance phase of the syndrome, whether other amino acids produce a similar lesion, and whether lysine in lower doses also produces acute renal failure. In the present study the authors show that lysine, when given in a dose of 600 mg/rat over 4 hours, produced persisting acute renal failure which at 48 hours was characterized morphologically by a picture similar to that in human "acute tubular necrosis"--little overt tubular necrosis, but a focal loss of individual tubular cells with regenerative changes and mitotic figures. Extensive hyaline cast formation was seen, particularly in the thin limbs of the loops of Henle, and these thin limb casts were shown to contain Tamm-Horsfall protein. Equivalent doses of glycine, arginine, and glutamic acid and lower doses of lysine produced no significant renal morphologic or functional changes.