THE biological significance of selenium as an essential micronutrient and toxic agent provides great impetus to the studies of its metabolism1. The distribution of selenium in the various organs of the body has been established2, and its metabolism—as determined by methods based on its excretion in faeces, urine and exhaled air—has been reported2–6. Isotopically labelled selenium (75Se) offers two methods for studying the metabolism of this element: direct measurement of radiation coming from an animal, and the difference between the injected dose and the cumulative activity in the excreta. The latter method is generally less satisfactory because of the errors caused by the inaccuracy in collecting the excreta in quantity.