Polyene antibiotics such as amphotericin and nystatin increase membrane permeability and thus increase the amount of oxygen consumed in active electrolyte transport. In isolated perfused rat kidneys, the polyenes produced extensive injury to the medullary thick ascending limb, a segment of the nephron with limited oxygen supply. This damage was prevented if reabsorptive transport was inhibited by ouabain. Cell death under these circumstances thus appears to be mediated by increased oxygen demand for transport activity.