The influence of diet ingredients and its supplementation with chosen B group vitamins on concentration of selenium in blood serum and tissues and activity of glutathione peroxidase in blood and liver of male rats was examined in the conducted experiment. The animals, aged 5 months, were divided into three groups and fed ad libitum with granulated mixes. Group I with basic mix containing among other things full grains, Group II with modified mix in which full grains were exchanged for wheat flour and in part with saccharose and Group III with modified mix supplemented in excess with vitamins B1, B2, B6 and PP. The experiment was conducted for six weeks during which the amount of consumed feeding stuff was calculated currently and once a week body mass of the animals was checked. When the experiment was finished the activity of GSH-Px was determined by spectrophotometric method in blood and liver whereas concentration of selenium in blood serum, muscles and in liver by fluorometric method. It was ascertained that the change of diet ingredients and its supplementation with chosen group B vitamins was in favour of lowering the amount of selenium in the examined tissues, and the decrease was not only the result of lower amount of the consumed element, but also of its increased usage, forced by the changes taking place under the influence of diet components and its supplementation.