Transport–dependent cell injury in the S₃ segment of the proximal tubule. Two distinct types of injury, cytoplasmic edema and cell fragmentation occur in the S₃ segment of the proximal tubule in isolated hypoxic perfused rat kidneys (Krebs–albumin medium gassed without O₂). The proportion of S₃ tubules with fragmentation strongly correlated with the GFR and urine output during the perfusion, and approached 100% when the GFR was increased by high perfusion pressure. Conversely, the fragmentation lesion was absent and the edema lesion extensive when tubular transport was inhibited by perfusion with hyperoncotic medium to prevent glomerular filtration or by addition of ouabain (10_-2M) to the perfusate. Polyene antibiotics increase membrane permeability and thus the work of active electrolyte transport. Perfusion with amphotericin (3 × 10 _-5M) or nystatin (200 U/mliter) in oxygenated medium also produced fragmentation in S₃. The lesion was prevented in the non-filtering kidney. Ouabain completely eliminated the cell fragmentation due to nystatin and significantly reduced that due to amphotericin. These results suggest that the injury of cell fragmentation is enhanced by transport activity and diminished when transport is inhibited. The edema lesion appears fundamentally different and more akin to lesions described in ischemia where tubular flow is absent, active transport is diminished, and the morphologic changes appear related to loss of cell volume regulation. The type of hypoxic damage exhibited by proximal tubular S₃ segments may therefore be conditioned by active ion transport of tubular cells.