Netilmicin (Sch 20569), a new semisynthetic aminoglycoside, was studied for its effects on kidney function and mechanisms by which it is handled by the kidneys. Measurements of glomerular filtration rate (GFR) and urinalysis in chronic rat studies indicated that the nephrotoxicity of netilmicin was remarkably less than that of gentamicin. Gentamicin caused a dose-related reduction in GFR in association with glucosuria and elevated fractional excretion of K(+). By contrast, high doses of netilmicin produced only slight reduction in GFR with increased fractional excretion of K(+) but without glucosuria. In separate experiments, rats were shown to excrete 71 to 90% of netilmicin or gentamicin in 24 h after daily intramuscular administration of doses of 20 or 40 mg/kg for 4 days. In acute experiments on anesthetized dogs, GFR and renal plasma flow were unaffected at serum levels of 11.0 +/- 0.6 mug/ml maintained by constant infusion of netilmicin for 5 h. The renal clearance of netilmicin was significantly correlated with GFR. The urinary output of netilmicin was 80.0 +/- 4.2% of the infusion rate and was independent of urine flow over the range of 0.04 to 0.33 ml/kg per min. Preferential accumulation of netilmicin occurred in the renal cortex; the cortex-serum and medulla-serum ratios were 9.9 +/- 1.2 and 4.2 +/- 0.6, respectively. In addition, the extraction ratio of netilmicin, which was lower than that of inulin, suggested that netilmicin reabsorption occurs in the proximal tubule and results in cortical accumulation. It is concluded that netilmicin, like gentamicin, is excreted by the dog kidney by glomerular filtration plus limited reabsorption. However, the new drug is characterized by low intrinsic nephrotoxicity in rats.