1. (75)Se as Na(2) (75)SeO(3) was administered orally to rats under different nutritional conditions. 2. The selenium found in the liver subcellular organelle fractions was present in at least three oxidation states: acid-volatile selenium, assumed to be selenide, zinc-hydrochloric acid-reducible selenium, assumed to be selenite, and higher oxidation states of selenium and organic derivatives, called selenate for convenience. 3. The proportion of the total selenium present as selenide present as selenide is susceptible to oxidation in vitro, which can be prevented by the addition of antioxidants in vitro. 4. The proportion of selenide is also directly related to the vitamin E status of the rats, and treatment of vitamin E-deficient rats with vitamin E results in an increase in the proportion of selenide. 5. Freezing the liver in situ before preparation of the organelle fractions did not alter the susceptibility of the selenide proportion to dietary vitamin E, indicating that the observed effects occur in vivo and not as a result of oxidation post mortem. 6. Intravenous administration of Na(2) (75)SeO(3), to rats whose alimentary tract was partially sterilized by neomycin treatment, gave a similar result to that in paragraph 4, indicating that the reduction of selenite to selenide probably occurs in vivo, and that intestinal micro-organisms are not responsible. 7. Treatment of vitamin E-deficient rats with silver produced a fall in the total (75)Se content of the liver, an effect only partially reversed by vitamin E administration. The proportion of the total selenium present as selenide was also lowered by the treatments with silver, and vitamin E significantly reversed this trend in most cases. 8. These results are consistent with the hypothesis that the active form of Se may be selenide and that the selenide may form part of the active centre of an uncharacterized class of catalytically active non-haem-iron proteins that are protected from oxidation in vivo by vitamin E.