The present work examines the effect of treatment of rats with graded doses of the aminoglycoside antibiotic gentamicin on the concentration of reduced glutathione (GSH) and diamine oxidase (DAO) activity in the kidney, and DAO activity, creatinine and magnesium (Mg) in the plasma. The animals were given the antibiotic intramuscularly in doses of 20, 40, and 80 mg/kg/day for 6 days, and were killed 24 hr after the last injection. In another experiment rats were injected intramuscularly with gentamicin at a dose of 80 mg/kg/day for 6 days and were killed 1, 7 or 14 days after the last injection, and the above parameters were measured. Gentamicin reduced the body weights of rats in a dose-dependent manner. The weight reductions were most marked on days 4, 5 and 6 of the treatment. The body weights gradually recovered on withdrawing of the drug, and by day 14, they were not significantly different from those of the controls. Gentamicin produced significant and dose-dependent decreases in the renal concentration of GSH. Seven and 14 days after withdrawing the drug, the GSH concentrations were still significantly below that of the controls. Plasma Mg concentrations were significantly decreased, and plasma creatinine concentrations significantly increased by gentamicin. These effects persisted 7 and 14 days after cessation of treatment. Plasma DAO activity was not detectable in the control or gentamicin-treated rats. In the renal cortex, the activity of the enzyme, measured 1, 7 and 14 days after the treatment, was not significantly different from that of the control. Histopathologically, the drug produced dose-dependent proximal renal tubular necrosis.(ABSTRACT TRUNCATED AT 250 WORDS)