As compared to rat liver microsomal arylamide N-hydroxylase both the dog and the human enzymes have lower affinity but higher activity. SKF525A, a well known effector of cytochrome P-450 dependent mixed function oxidases, activates the hepatic N-hydroxylase of all three species. This effect is concentration dependent and tends to plateau at 50 X 10(-6)M. As previously demonstrated with rat liver microsomes, the ring-hydroxylated, non-toxic, metabolites of 2-acetylaminofluorene, interact with the N-hydroxylating enzyme. These interactions are both compound- and species-specific. The most striking differences are seen with the paraphenolic product which activates the rat, does not affect the dog and inhibits the human liver enzyme. In the liver of this last species, that compound is the main metabolite of 2-acetylaminofluorene (2-AAF).