Acetaminophen (APAP) produces proximal tubular necrosis in Fischer 344 (F344) rats. Recently, p-aminophenol (PAP), a known potent nephrotoxicant, was identified as a metabolite of APAP in F344 rats. The purpose of this study was to determine if PAP formation is a requisite step in APAP-induced nephrotoxicity. Therefore, the effect of bis(p-nitrophenyl) phosphate (BNPP), an acylamidase inhibitor, on APAP and PAP nephrotoxicity and metabolism was determined. BNPP (1 to 8 mM) reduced APAP deacetylation and covalent binding in F344 renal cortical homogenates in a concentration-dependent manner. Pretreatment of animals with BNPP prior to APAP or PAP administration resulted in marked reduction of APAP (900 mg/kg) nephrotoxicity but not PAP nephrotoxicity. This result was not due to altered disposition of either APAP or acetylated metabolites in plasma or renal cortical and hepatic tissue. Rather, BNPP pretreatment reduced the fraction of APAP excreted as PAP by 64 and 75% after APAP doses of 750 and 900 mg/kg. BNPP did not alter the excretion of APAP or any of its non-deacetylated metabolites nor did BNPP alter excretion of PAP or its metabolites after PAP doses of 150 and 300 mg/kg. Therefore, the BNPP-induced reduction in APAP-induced nephrotoxicity appears to be due to inhibition of APAP deacetylation. It is concluded that PAP formation, in vivo, accounts, at least in part, for APAP-induced renal tubular necrosis.