Activities of the renal and hepatic microsomal enzymes responsible for the N-hydroxylation and mutagenic activation of 3-methoxy-4-aminoazobenzene (3-MeO-AAB) were examined in male mice, rats, hamsters and guinea pigs. In all these rodent species, hepatic microsomes showed definite N-hydroxylation of 3-MeO-AAB, whereas the renal activity was detected only in mice. The hepatic enzyme responsible for N-hydroxylation of 3-MeO-AAB (3-MeO-AAB N-hydroxylase) was induced in all species except mice by phenobarbital and selectively in mice and hamsters by 3-methylcholanthrene, whereas these cytochrome P450 inducers did not affect the renal enzyme in mice, rats or hamsters. In individual microsome samples, activities for N-hydroxylation and mutagenic activation of 3-MeO-AAB correlated well. These results indicate that the renal and hepatic enzymes responsible for the metabolic activation of 3-MeO-AAB differed among different species of rodent animals in terms of their activity and inducibility with cytochrome P450 inducers.