The metabolism of 2‐acetylaminofluorene (AAF)in vitroand the urinary excretion of a single ip dose of AAF were studied in weanling rats maintained on an Se‐deficient diet or the same diet supplemented with 0.5 ppm Se. Hepatic microsomes isolated from Se‐supplemented rats generated greater amounts of noncarcinogenic phenolic metabolites of AAF than did microsomes from Se‐deficient animals. Under the same conditions, the production of the proximate carcinogenic metaboliteN‐hydroxy‐AAF was very low and no difference was detected between the two groups of rats. Microsomes from Se‐deficient rats exhibited moderately higher glucuronyltransferase activity. However, no difference was found between the two dietary groups with respect to hepatic soluble sulfotransferase activity.