The review deals with the most important aspects of selenium in rumina
nts. The absorption, distribution, metabolic transformation, secretion
in milk and placental transmission of selenium are described. Seleniu
m is absorbed in the lower parts of the small intestine, then it is tr
ansported to the liver bound to proteins and in a free form. From the
liver, selenium is reverted to the blood stream bound to alpha and gam
ma globulines, where it is incorporated to selenoproteins, chiefly sel
enocistein into gluthatione peroxidase (GSH-Px). The incorporation int
o proteins is mediated by gene products called SelA, SelB, SelC and Se
lD. After absorption, selenium metabolism in ruminant is similar to no
n-ruminant animals. Factors that interfere with selenium metabolism, l
ike chemical forms of selenium, level of selenium in blood and tissue
before supplementation, presence of some minerals and aminoacids in th
e ration, calcium level in the diet, and concentration of selenium in
the ration, are described.