The nephrotoxicity of hydroxygentamicin and amikacin was examined in young adult Fischer 344 rats. Serum creatinine (SCr) and urea nitrogen (BUN) levels were not significantly affected following sc injection of 80 or 160 mg/kg/day of hydroxygentamicin for 15 days. However, 250 mg/kg of amikacin produced significant increases in both parameters and in kidney/body weight ratios. The ratios were also significantly increased after 80 or 160 mg/kg of hydroxygentamicin, but kidneys of rats receiving amikacin were considerably heavier than those of rats treated with hydroxygentamicin. The antibacterial potency of 250 mg/kg of amikacin is comparable to that of 100 mg/kg of hydroxygentamicin. Additional studies, directly comparing hydroxygentamicin, a mutational biosynthetic, with gentamicin or netilmicin, all at 40, 80, and 160 mg base/kg, and incorporating renal function parameters as well as SCr, BUN, organ weight, tissue concentration, and kidney histopathology, revealed a characteristic pattern typical of aminoglycoside nephrotoxicity in mature adult male rats. In most parameters, values in rats given hydroxygentamicin or netilmicin were normal and comparable to those in controls, but kidney/body weight ratios were significantly increased at high doses. However, kidneys of rats medicated with gentamicin at comparable doses were considerably heavier than those of hydroxygentamicin-treated rats. Significant nephrotoxicity also was seen in rats given low doses of gentamicin or netilmicin. Eosinophilic granulation and vacuolization of renal proximal tubular epithelium, interstitial inflammation, and tubular dilation were observed microscopically with all three drugs in the following descending order of severity: gentamicin greater than netilmicin greater than hydroxygentamicin. The effects on proximal tubular epithelial cells following treatment with amikacin, netilmicin, or hydroxygentamicin correlated reasonably well with renal drug concentrations, but drug concentrations of gentamicin, which produced the most extensive kidney injury, were lower than those of the other three aminoglycosides. Elevated SCr or BUN were indicative of the presence of nephrosis, but early stages of tubular epithelial degeneration were not predicted by increases in BUN or SCr. Although minimal or mild nephrosis was seldom predicted by polyuria, proteinuria, or changes in osmolality, effects observed in renal function parameters usually correlated well with renal histopathology. However, a decrease in osmolality correlated best with enlarged kidneys and changes in renal morphology.