The in vitro metabolic N-oxidation of 1- and 2-naphthylamine, 4-biphenylamine, 2-fluorenylamine and 3-dibenzofuranylamine was investigated with intact dog bladder, whole intact bladder mucosa and microsomes prepared from this tissue. Very low levels of metabolic N-oxidation of these carcinogenic amines were detected with these tissue preparations using ferrihemoglobin formation in dog erythrocytes. No N-oxidation by these tissue preparations was observed using GLC. The concentrations of the N-oxidized metabolites observed in the urine of dogs in vivo exposed to these amines suggests that N-oxidation takes place predominately in the liver and the bladder plays, at most, a minor role in the formation of these presumed proximate urinary carcinogens.