The catalytic efficiences of cytochrome P450 (P450)-mediated N-oxidation of the 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) by recombinant human P450 1A2 were 10-19-fold higher than rat P450 1A2, while methoxyresorufin O-demethylation activity was comparable for both P450s. Similar findings were observed with rat and human liver microsomal samples. Interspecies differences in P450 enzyme expression and catalytic activities may be significant and must be considered when assessing human health risk.