Glutathione peroxidase (GSH-Px) contains 4 selenium atoms/molecule; its activity is increased by selenium dietary intake. The enzyme destroys H2O2 and organic hydroperoxides, contributing to the integrity of biological membranes. GSH-Px activity increased (+100%) in washed platelets of rats administered selenium (0.3 ppm given as sodium selenite) for 60 days from 10.44 +/- 1.10 U/g protein (control rats fed a standard diet) to 20.50 +/- 1.21 U/g protein (mean +/- SE; P less than 0.001). GSH-Px in washed erythrocytes was also stimulated (+70%) after 80 days of selenium dietary intake from 11.60 +/0 0.82 U/g Hb to 19.74 +/- 0.94 U/g Hb (P less than 0.001). Malondialdehyde (MDA), the typical breakdown product of peroxidized lipid and a suitable indicator of platelet prostaglandin production, increased from 0.343 +/- 0.035 nM/3 X 10(8) platelets (control) to 0.478 +/- 0.052 nM/3 X 10(8) platelets after 30 days of selenium treatment (P less than 0.05) and to 0.527 +/- 0.051 nM/3 X 10(8) platelets after 80 days (P less than 0.01). MDA was measured by the thiobarbituric acid method after stimulation with 25 X 10(-4) M arachidonic acid. It is concluded that platelets are very rich in GSH-Px, i.e., activity is greatly increased by oral administration of selenium and that the synthesis of prostaglandins is stimulated too.