Certain azo compounds and their N-oxides have been prepared and characterized by spectroscopic methods. A HPLC method for the quantification of azo-N-oxide metabolites of those azo compounds is described. The enzymic N-oxidation of azo compounds in vitro has been demonstrated as a general metabolic pathway. In the case of the mixed aryl-alkyl azo compound, only one N-oxide was detected. The identity of N-oxide metabolites has been confirmed by CG/MS. Azo-N-oxidase activity was detected principally in the liver, but also in lung, heart and kidney whole organ homogenates. Studies using subcellular fractions showed that azo N-oxidase activity resides mainly in the hepatic microsomal fraction. The amount of N-oxide formed under optimised incubation conditions by hamster microsomal preparations is greatest with omega, omega'-azoxytoluene (AXT) and least with azoxybenzene (AXB); while omega-(phenylazoxy)toluene (PAXT) is intermediate.