Sprague-Dawley rats given gentamicin from 10 to 70 mg/kg/day for 9 days showed a linear decrease in glomerular filtration rate with increasing dose, paralleled by histologic changes of acute tubular necrosis and cast formation only at the higher doses. Nephrotoxicity was correlated with the peak, rather than trough, serum gentamicin levels in this study, suggesting that it is the mean level of gentamicin over time that determines renal injury. The polyuria caused by gentamicin resulted mainly from a tubular concentrating defect rather than enhanced sodium or osmolal excretion and may be explained by the finding of a predominance of casts in the medullary thin limbs of the loops of Henle. No effect of gentamicin on the activity of cortical or medullary sodium-potassium adenosine triphosphatase was found to account for the modest sodium wasting. Concurrent administration of sodium1 cephalothin decreased the renal toxicity of gentamicin at high doses, an effect not explained by the added sodium or nonreabsorbable anion.