The effects of dietary protein on renal function and structure, both prior to and after initiation of daily gentamicin treatment, were investigated. Male Sprague-Dawley rats were pair-fed on low-protein (LP, 5%), normal-protein (NP, 20%), or high-protein (HP, 60%) diets for 10 days prior to gentamicin treatment. Gentamicin was administered as daily subcutaneous injections (150 mg/kg) for 6 days. Immediately after beginning daily gentamicin injections some of the rats on NP diets were switched to LP or HP diets, and some of the rats on HP diets were switched to LP diets. Renal function was monitored by evaluating serum creatinine levels and 24-h urine volumes; renal histology was evaluated by light and electron microscopy; and gentamicin uptake was determined using radioimmunoassay. Our findings indicate that conditioning to higher dietary protein prior to gentamicin administration results in less uptake of gentamicin by the kidneys. If rats on HP diets are placed on LP coincident with gentamicin administration, there is a significant improvement in survival. Switching rats from NP to LP protein coincident with gentamicin administration does not improve renal function, histology, or survival. However, switching rats from NP to HP coincident with gentamicin administration significantly increases mortality. Maintaining rats on LP both prior to and after gentamicin administration results in a significant improvement in survival but does not improve renal Junction. These results indicate that dietary protein both prior to and following the administration of gentamicin can significantly affect the nephrotoxicity of gentamicin.