Magnesium and selenium belong to important bioelements. Magnesium is the second most abundant intracellular macroelement, which takes part in the metabolism of carbohydrates, nucleic acids, protein and lipids. Selenium is an essential microelement, whose deficit has been stated in many different pathological states. Much research on safe and effective selenium supplementation has been performed for the last fifty years but the results still remain unsatisfactory. The aim of our study was to investigate the influence of inorganic sodium selenite Na2SeO3 and two selenoorganic compounds synthetized at our chair on magnesium concentrationin tissues of adolescent male Wistar rats. Inorganic selenite was administered as a water solution, where as organic compounds: 4-(o-tolilo)-selenosemikarbazyd of 2-chlorobenzoicacid of a chain structure (ORG-C) and 3-(o-chlorobenzoylamino)-2-(o-tolylimino)-4-methyl-4-selenazoline of a ring structure (ORG-R) were suspended in emulsion (oil, arabicgum and water). Selenium compounds were given to rats at a dose of 5⋅10–4 mg Se g–1 b.w. once a day for a period of 10 days. The control group was treated with saline. The administration was performed with use of a stomach tube. In comparison to the control group, selenium supplementation caused decrease in magnesium concentration in kidney and lung tissues, but did not cause any changes in the brain and heart muscle. In the liver and spleen it was only ring selenazoline that affected magnesium concentration, increasing it in the liver and decreasing in the spleen. In thefemoral muscle it was only the selenosemicarbazide chain that exerted the significant effectc a using a decrease in Mg concentration vs the control group. Selenium supplementation influences the tissue magnesium concentrations dependingon tissue and structure of the supplement. Irrespective of the administered compound, it lowered magnesium in kidneys and lungs but caused no changes in the brain and heart muscle. In the liver, spleen and femoral muscle, alterations in the magnesium concentration were dependent on the provided supplement.