The principal role of cytochrome p-450 in mixed function oxidase reactions includes hydroxylation of vavious drugs, fatty acids, steroids, pesticides, and carcinogens. The effect of AAF, β-NF with vitamin A, and α-tocopherol on hepatic micros-omal cytochrome p-450 content in young rats were investigated. The result is as follows; 1) The administrations of AAF, β-NF increased contents of hepatic microsomal cytochrome p-450. 2) The increased amount of hepatic microsomal cytochrome p-450 include by AAF declined wien vitamin A, α-tocopherol and β-NF were given. 3) AAF-ring-hydroxylation decreased in AAF group, but N-hydroxylation increased, compared to control group. 4) The administration of β-NF or β-NF and AAF increased both ring-hydrox-ylation and N-hydroxylation. 5) AAF-ring-bydroxylation increased when AAF was administrated with vitamin A, α-tocopherol, and β-NF compared to AAF group.