To determine critically the selenium (Se) requirement for weanling female rats, we used glutathione peroxidase (GSH: H2O2 oxidoreductase, EC 1.11.1.9) (GPX) mRNA and a number of other parameters to assess Se status. Rats were fed a basal torulayeast diet (0.007 micrograms Se/g) supplemented with Se as Na2SeO3 in graded levels from 0 to 0.3 micrograms Se/g diet for 32 d (3 rats/group). Selenium supplementation had no effect on growth, showing that the Se requirement for growth is less than 0.007 micrograms Se/g diet, whereas other parameters showed significant increases with Se supplementation. In rats fed the Se-deficient basal diet, liver Se concentration was 4 +/- 0%, plasma GPX activity was 8 +/- 1%, erythrocyte GPX activity was 40 +/- 3%, liver GPX activity was 2 +/- 1%, and liver GPX mRNA levels were 11-17% of the levels in rats fed 0.1 micrograms Se/g diet. Liver Se concentration and GPX activity in plasma, erythrocytes and liver all reached a plateau breakpoint at or near 0.1 micrograms Se/g diet, indicating that the dietary Se requirement for maximal GPX activity in growing female rats is 0.1 micrograms Se/g diet. Liver GPX mRNA levels reached the plateau breakpoint at 0.05 micrograms Se/g diet, showing that the minimum dietary Se requirement for maximal GPX mRNA levels in female rats is half of the Se requirement for maximal GPX activity. This experiment demonstrates that GPX mRNA can be used to determine the dietary Se requirement; the gap between the dietary Se necessary for maximal GPX mRNA and that for maximal GPX activity may represent an evolutionarily derived biological margin of safety.