After an initial episode of acute tubular necrosis, apparent resistance to gentamicin nephrotoxicity develops in rats during prolonged drug administration. The authors studied this phenomenon by examining the autoradiographic distribution of 3H-gentamicin and 3H-thymidine during 5 weeks of gentamicin treatment and by analyzing renal structure and function after a 12-week course of treatment. These studies show that regenerating cells exclude gentamicin, but concentrate it again after maturation, and that the rate of thymidine incorporation is still high well after recovery from acute toxic injury. After 12 weeks of gentamicin, the glomerular filtration rate was only modestly diminished, whereas in vitro cortical organic ion transport was substantially impaired. Light and electron microscopy demonstrated all phases of injury and recovery among cells of most proximal tubules and evidence of chronic tubulointerstitial disease. It is concluded that "resistance" to gentamicin is a state of persistent tubular cell injury obscured functionally by preservation of the glomerular filtration rate and histologically by asynchrony of cell necrosis and regeneration.