In this report the ability of nuclei to carry out mixed function oxidase activity has been discussed. The properties of the nuclear system have been contrasted with those of microsomes and the localization of the nuclear mixed function oxidase has been definitively established. In addition, the nuclei have been shown to contain an inducible O-deethylase and epoxide hydrase(s). Finally, nuclei have been demonstrated to catalyze the formation of benzo[a]pyrene syn- and anti-diolepoxides, ultimate carcinogenic forms of this polycyclic hydrocarbon.