The authors performed another controlled trial in 33 top swimmers (16 girls and 17 boys) in order to make evident some acute and chronic effects (antioxidant) of selenium. Lipid peroxides (MDA-malondialdehyde), nonproteic--SH (essential glutathione) in the serum and blood lactate had been recorded initially on basal conditions and after 2 h endurance training (swimming) accompanied by a per oral administration of 150 micrograms selenium (respectively placebo); one week later we applied the cross-over method. In another trial we continued the treatment with 100 micrograms selenium daily for 14 days (n = 9), respectively placebo (n = 7) and then we applied the crossover method for another 14 days. The above biochemical parameters were recorded initially at rest, under basal conditions, after 14 days of treatment and again after 14 days of treatment, when crossing-over. No significant changes were noticed after a single dose + 2 h hard training, both under selenium and placebo treatment and also when the cross-over method was applied. Fourteen days of selenium treatment induced significant changes of lipid peroxides (especially when the subjects came after placebo) and nonproteic--SH, compared to placebo, changes which support the idea of some antioxidant effects of selenium which might be useful in endurance athletes.