The biological effect of selenate and selenite was compared in selenium-depleted rats by using both serum selenium concentration and glutathione peroxidase activity as an indicator of body selenium status. A single oral dose of selenium (125 µg/kg body weight) as sodium selenate or sodium selenite increased serum selenium concentration and glutathione peroxidase activity significantly (p < 0.001). The effect of selenate and selenite on serum selenium and glutathione peroxidase activity was similar. Serum selenium concentration correlated positively with serum glutathione peroxidase activity both before (r = 0.815; p < 0.001) and after (r = 0.800; p < 0.001) treatment. These results indicate that the biological availability of selenate and selenite is similar