Strain differences in susceptibility of rats to acetaminophen (APAP)-induced nephrotoxicity have been reported previously. Young adult male Fischer 344 (F344) rats are susceptible, whereas weight-matched Sprague-Dawley (SD) rats are not susceptible to APAP nephrotoxicity. Susceptibility to APAP nephrotoxicity is also age dependent, at least in F344 rats. Middle-aged (12-15 months old) male F344 rats are more susceptible to APAP-induced nephrotoxicity than are young adult (2-4 months old) males. APAP nephrotoxicity in aging SD rats has not been evaluated. The present studies were designed to define strain differences in the nephrotoxicity of APAP and p-aminophenol (PAP), a nephrotoxic metabolite of APAP, using 2-, 3-, and 9- to 12-month-old F344 and SD rats. At 2 months of age, F344, but not SD, rats were susceptible to APAP-induced nephrotoxicity. However, at 3 months of age, strain differences were less marked, as susceptibility to APAP nephrotoxicity appeared to increase between 2 and 3 months of age only in SD rats. By 9-12 months of age, susceptibility to APAP nephrotoxicity was comparable in F344 and SD rats. No age- or strain-related differences were observed in the excretory pattern of urinary APAP and metabolites that might explain the increased susceptibility of aging rats to APAP nephrotoxicity. Strain differences in age-matched rats were not marked for PAP-induced nephrotoxicity. Susceptibility of both 3- and 12-month-old F344 and SD rats to PAP-induced nephrotoxicity was greater compared to strain-matched 2-month-old rats. In both F344 and SD rats, PAP nephrotoxicity increased only modestly between 3 and 12 months of age, indicating that increased susceptibility to PAP probably does not play a major role in the age-dependent increase in APAP nephrotoxicity. Thus, strain differences in APAP nephrotoxicity decrease with advancing age. The mechanisms mediating the increased susceptibility to APAP nephrotoxicity in middle-aged rats are not known but may relate, at least in part, to age-dependent differences in pharmacokinetics. The present study highlights the importance of considering the age of rats when evaluating drug toxicity. Even in young adult rats, subtle maturational changes in drug metabolism and/or disposition may occur, making toxicological evaluation in weight-matched rats of different strains and ages inappropriate.