Determination of oxycodone metabolites excreted in urines and feces of several mammalian species was studied by use of the tritium labeled compound. It was found that the radioactivity was excreted more in the urines than in the feces in rabbits, guinea pigs and mice, but not in rats, who eliminated it equally into the urines and feces. Most of the radioactivity was excreted in 48 h after the administration. Seven metabolites, oxymorphone, 6 alpha-oxycodol, 6 beta-oxycodol, 7 beta-hydroxy-6 beta-oxycodol, noroxycodone, oxycodone N-oxide, and 6 alpha-oxycodol, N-oxide, as well as well as unchanged oxycodone, were found to be excreted into urines of rabbits in both free and conjugated forms except two N-oxides, which were found only in the free form. It was discussed that analgesic effect of oxycodone would mainly be attributable to the oxycodone itself, but not to the metabolites.