Chinese hamster ovary (CHO) cells were exposed to 2-acetylaminofluorene (2-AAF) and 2-aminofluorene (2-AF), and several of their N-oxidized metabolites in order to study the mechanisms by which arylamides and arylamines produce mutations in mammalian cells. The number of mutations induced at the hypoxanthine-guanine phosphoribosyl transferase locus by each compound (mutants/ lo6 CHO cells/nmol compoundlml) was estimated to be: N-acetoxy-ZAAF, 310; Nhydroxy-ZAF, 3; N-hydroxy-2-AAF (with and without hepatic S9 activation), 0.7; 2-AAF (with S9), 0.1; and 2-AF (with S9), 0.09. With each compound, DNA adducts were also identified and quantified, and in all cases the major adduct was N-(deoxyguanosin-8-y1)-2-AF. 2-AAF and N-hydroxy-2-AAF also formed minor amounts of N-(deoxyguanosin-8-y1)-2-AAF and 3-(deoxyguanosin-N2-y1)-2-AAF. The relationship between mutation induction and adduct formation for each of the derivatives was similar to that previously reported for N-hydroxy-2-AF. Inclusion of the deacetylase inhibitor, paraoxon, reduced the mutagenicity of 2-AAF, N-hydroxy-2-AAF and N-acetoxy-2-AAF, and the DNA adducts produced by N-acetoxy-2-AAF to background levels. Acetyl coenzyme A increased the mutations and CHO cytosol-mediated DNA binding of N-hydroxy-ZAAF, but did not substantially increase these responses from N-hydroxy-2-AF. N-Hydroxy-2-AAF was not detectably metabolized by CHO cells. Taken together, these data indicate that CHO cells metabolized N-acetoxy-2-AAF to a reactive derivative by