Selenium (Se) is a trace element of importance implicated in many processes such as antioxidant mechanisms, immune response, fertility in males and females, protection factor for cardiovascular diseases and some cancer processes (see review by Rayman 2000). There are mainly two modes of action for Se either by a direct implication of metabolites such as the methylselenol or by the specific effects of selenoproteins in which Se is an essential compound in the active site as selenocysteine. Glutathion peroxidase (GPx), thioredoxine reductase, iodothyronine deionidase are some of the selenoproteins. Most feedstuffs produced in Western Europe are low in Se. So, Se has to be supplemented in animal diets. This could be done either on an organic or on an inorganic form. Selenate contained in fertilizers is naturally transformed in organic forms such as selenomethionine by the plants. The aim of the present work was to assess, in exercised horses, the effects on the antioxidant status of feedstuffs produced with Se enriched fertilizers.