An N-acetylation polymorphism is described that is expressed toward arylamine carcinogens in tumor target organs of an inbred rat model. High levels (rapid acetylator phenotype) of arylamine carcinogen N-acetyltransferase activity were observed in kidney, colon, prostate and urinary bladder cytosols derived from Fischer (F-344) inbred rats, the strain most commonly used for tumor bioassay studies and the strain most particularly used in arylamine-induced colon and prostate cancer studies. Significantly lower (slow acetylator phenotype) levels of arylamine carcinogen N-acetyltransferase activity were observed in corresponding tissue cytosols derived from Wistar-Kyoto inbred rats. Intermediate levels of arylamine carcinogen N-acetyltransferase activity significantly different from both the parental strains were observed in F1 hybrids of the parental strains, consistent with codominant expression of two alleles at a single gene locus. The arylamine substrates exhibiting the acetylator phenotype-dependent N-acetyltransferase activities included p-aminobenzoic acid, p-aminosalicylic acid, p-phenetidine, p-aminophenol, 2-aminofluorene, 3,2'-dimethyl-4-aminobiphenyl, beta-naphthylamine and 4-aminobiphenyl, but not procainamide. Highest levels of arylamine carcinogen N-acetyltransferase were expressed consistently in colon cytosol, but expression of the N-acetylation polymorphism toward arylamine carcinogens was observed in each (kidney, colon, prostate and urinary bladder) of the tumor target organs. The expression of the N-acetylation polymorphism in tumor target organs suggests that the inbred rat model will be useful in assessing the role of acetylator phenotype in arylamine-induced cancers of the colon and prostate.