Organophosphorus insecticides with substituted or unsubstituted 4-nitrophenyl ester structure were examined for their metabolism in vitro by microsomes of rat and rabbit livers and of housefly abdomens. Compounds studied included methyl parathion, parathion, n-propyl parathion, iso-propyl parathion, dicapthon, Sumithion, Chlorthion and O-ethyl O-4-nitrophenyl phenylphosphonothioate and their P O analogs. All P S compounds were metabolized via two oxidative pathways, activation to P O analogs and cleavage at the aryl phosphate bond. These reactions occurred only in the presence of NADPH 2 and O 2. The oxidative cleavage also occurred with one P O compound, n-propyl paraoxon.