To evaluate the nephrotoxicity of the new aminoglycoside, dibekacin, relative to gentamicin, we administered both drugs in doses of 40 and 120 mg/kg per day to male and female Fischer 344 rats. At sacrifice, we determined serum creatinine, in vitro renal cortical uptake of 14C N-methyl nicotinamide and para-aminohippurate, renal cortical antibiotic concentrations, and real histology. Dibekacin and gentamicin were similar in overall toxicity. Dibekacin differed from gentamicin and other aminoglycosides (1) in failing to cause early transient stimulation of para-aminohippurate uptake in male rats and (2) in the location of histologic damage at the 120 mg/kg dose. We conclude that: 1) in this model, dibekacin is comparable to gentamicin in nephrotoxic potential, and 2) the lack of early stimulation of para-aminohippurate uptake in male rates after dibekacin treatment may be related to greater initial injury to the late proximal tubule than is caused by gentamicin.