Netilmicin, gentamicin, tobramycin, amikacin, kanamycin, streptomycin, and sisomicin were given daily for 15 days to groups of rats at three dosage levels corresponding to 10, 15, or 25 times the dose recommended for humans on a weight basis. Decreased urinary osmolality and increased urinary excretion of protein and beta-N-acetyl hexosaminidase were dose-related features of nephrotoxicity. Decreased tubular resorption of glucose and phosphate were observed with the most toxic regimens after extensive renal damage had occurred. All aminoglycosides accumulated in renal tissue; however, the concentration of drug in the renal cortex at the time the rats were killed was not useful for the prediction of renal impairment. Streptomycin and netilmicin exhibited a flat dose-reponse curve with respect to histological damage, as compared with the curves for the other drugs. Results of studies of creatinine clearance and examination of renal tissue suggested the following order of increasing toxicity of the treatment regimens: (1) 0.9% NaCl and uninjected controls; (2) streptomycin; (3) netilmicin; (4) tobramycin; (5) sisomicin, amikacin, and kanamycin; and (6) gentamicin.