We will conduct a double-blind, randomized, placebo-controlled clinical study of selenium supplementation in the form of L-selenomethionine (SM)(200 µg/day) and selenium-enriched yeast (SY) (200 µg/day and 285 µg/day) for 9 months; the 285 µg/day SY is selected to deliver an equivalent selenium as in 200 µg/day SM to healthy men. The variability of SM in SY that will be used here is less than 3% and it accounts for 70.5% of the SM content in SY. As primary endpoints, we will determine the effects of these two forms of selenium on plasma levels of selenium and its metabolites as well as biomarkers of oxidative stress at several time points. As a secondary endpoint, the effect of these two forms of selenium on plasma PSA levels will be examined.