The tubular epithelial cells located in the renal medulla are normally working in a hypoxic milieu. In isolated rat kidneys perfused with a cell-free medium, the medullary thick ascending limbs of Henle's loop are selectively and reproducibly injured by the imbalance between oxygen demand and supply in this area. Hypoxic lesions rapidly progress from reversible to irreversible forms of cell damage. Reversible injury consists of chromatin margination and mitochondrial swelling, which can disappear upon restoration of an adequate balance of oxygenation. Irreversible injury consists of nuclear pyknosis and cytoplasmic fragmentation, lesions which persist after re-oxygenation or even progress to cell death. Reversible and irreversible phases of hypoxic injury in this distal tubule segment are comparable to, but different from, those previously defined for the proximal tubule.