Gentamicin, because it is stored in renal tissues, can prevent acute retrograde pyelonephritis. Since different aminoglycosides accumulate and persist to various degrees in the kidney parenchyma, the prophylactic activities of gentamicin, tobramycin, amikacin, and netilmicin were compared. The four antibiotics were given intramuscularly to rats 3 days before initiating ascending unilateral pyelonephritis with Escherichia coli. Despite different degrees of renal accumulation at the time of infection (tobramycin and amikacin accumulated significantly less), all four aminoglycosides displayed similar protection against ascending pyelonephritis. This protection was conferred in the absence of active antibiotic detectable in the urine and was therefore attributed to antibiotic stored in the renal parenchyma. Those animals that developed pyelonephritis despite aminoglycoside prophylaxis had less severe acute kidney infection and inflammation. This resulted 3 months later in an almost complete protection against renal scarring (chronic pyelonephritis). These results in rats suggest that renal accumulation and persistence of aminoglycosides may be used to advantage in the prophylaxis or in the treatment of kidney infections.