Selenium is one of the most important elements in poultry nutrition. It is an important antioxidant and influences the absorption of some vitamins. Its effect depends on the form and concentration in the feed. Organic sources are more efficient than inorganic. In some respects, such as the activity of glutathione peroxidase, inorganic sodium selenite is equivalent to organic selenium forms. The total selenium concentration in the mixed feed shall not exceed 0.5 mg/kg.