The influence of vancomycin on tobramycin nephrotoxicity was assessed in male Fischer rats. Treatment groups included controls receiving diluent and groups receiving vancomycin alone at a dosage of 200 mg/kg (body weight) per day, tobramycin alone at a dosage of 80 mg/kg per day, and a combination of vancomycin and tobramycin at the above dosages. All regimens were injected on a twice-a-day schedule. The animals were sacrificed on days 1, 3, 10, 14, 17, and 21. When compared with controls, animals receiving vancomycin alone exhibited no detectable renal toxicity. Compared with the case with controls, tobramycin alone was toxic, as manifested by lower mean animal weights, increased blood urea nitrogen concentrations on days 14 and 17 (P less than 0.005), increased serum creatinine concentrations on days 17 and 21 (P less than 0.005), and the presence of renal cortical tubular necrosis and regeneration. When compared with tobramycin alone, the combination of vancomycin and tobramycin caused earlier and more severe toxicity. By day 10, the magnitude of weight loss, the rise in blood urea nitrogen, and the increase in serum creatinine concentration were all greater in the rats given the combination of vancomycin plus tobramycin than in the animals given tobramycin alone (P less than 0.005). In addition, there was more proximal tubular necrosis and regeneration in rats given vancomycin plus tobramycin compared with those given tobramycin alone. In this animal model, vancomycin alone caused no detectable renal injury, tobramycin alone produced minimal proximal tubular damage, and the combination of vancomycin and tobramycin resulted in a greater degree of kidney injury than observed with tobramycin alone.