Until recently, studies of the function of selenium focused on the selenoenzyme, glutathione peroxidase. However, the recognition that several metabolic effects of selenium are not associated with glutathione peroxidase has forced a re-evaluation of the function of this enzyme and the element. Hepatic glutathione peroxidase contains a significant percentage of the regulated selenium in the rat and is more sensitive to selenium deficiency than other selenoproteins. Thus, in addition to its enzymatic activity, it might have a storage function for the element. Another selenoprotein, designated selenoprotein P, has been found in rat plasma and has been quantitated. Its function is not yet known, but it has been postulated to be a transport protein for selenium and a defense against oxidant stress. Understanding the nutritional effects of selenium will require better characterization of glutathione peroxidase, selenoprotein P and other selenoproteins.