To explore whether bacteremia potentiates gentamicin nephrotoxicity, we injected rats with either 1 X 10(9) Escherichia coli (E. coli), Pseudomonas aeruginosa, or Staphylococcus aureus, and then gave them gentamicin, 100 mg/kg. Renal injury was assessed over the next 24-48 h. Staphylococcus/gentamicin or gentamicin alone induced no renal injury. However, E. coli/gentamicin and Pseudomonas/gentamicin caused acute renal failure (severe azotemia; tubular necrosis; cast formation). This effect was not due to acute reductions in arterial blood pressure or renal blood flow, it could be reproduced by substituting nonviable for viable gram-negative organisms, and it was associated with increased renal gentamicin uptake. E. coli without gentamicin induced only mild azotemia and no tubular necrosis. Endotoxin-tolerant rats were significantly protected against the E. coli/gentamicin nephrotoxic interaction. We conclude that gram-negative bacteremia and gentamicin exert synergistic nephrotoxicities; and that this effect is mediated, at least in part, by endotoxin and in part by increased renal gentamicin uptake.