Selenium, as an important trace element, is part of selenoproteins with growth-modulating properties and multifactorial mechanisms of action. The biological effects of Se depend on the dose and chemical form in which it enters the body. Selenium nanoparticles have unique characteristics, including small size, large surface area, solubility and multifunctionality. The inclusion of phytonutrient-functionalized nanoparticles in feed is expedient due to solubility, protection against oxidation and enzymatic degradation, increased residence time, and increased bioavailability. Biogenic selenium nanoparticles, obtained by "green" synthesis methods with the participation of microorganisms and plant extracts, are biocompatible and less toxic compared to inorganic selenium compounds. This study was aimed at studying the effect of different forms of selenium in a complex with a probiotic (L. plantarum) on metabolic processes in the body of broiler chickens in order to provide an experimental basis for the effective use of selenium-containing feed additives. A total of 200-day-old broiler chickens of the Cobb 500 cross were divided into four groups by the analog method: K (PC), sodium selenite (0.3 mg Se/kg feed) + probiotic (L. plantarum) (2nd group); biogenic nanoselenium (0.3 mg Se/kg feed + L. plantarum (3rdgroup); nanobioconjugate of selenium with flavonoids of onion peel (0.3 mg Se/kg feed + L. plantarum (group 4). Supplements of selenium and nanoselenium in a complex with a probiotic increased the content of protein in blood serum, in the 3rd and 4th groups this difference was significant (p<0.05). Compared to the control, the activity of alkaline phosphatase increased in all experimental groups, but in broilers of the 4th group, the increase was probable (p<0.05). The content of total lipids, uric acid and activity of aminotransaminases probably did not differ in the experimental groups. The content of creatinine in blood serum and TBC-active products in the liver of birds of the 3rd and 4th groups was lower (p<0.05), but the amount of reduced glutathione (GSH) in the liver tissues was probably higher. It has been established that selenium nanoparticles synthesized with the participation of green technologies are non-toxic, biocompatible, have antioxidant and hepatoprotective properties. The obtained results are an important guideline for the use of nanoselenium conjugates synthesized by the innovative "green" method with flavonoids of onion peel in a complex with a probiotic strain of lactobacilli (L. Plantarum) as a functional feed additive to improve the metabolic processes of the broiler body and can be included in the composition feed additives for poultry. Key words: nanotechnology, selenium nanoparticles, "green" synthesis, quercetin, onion peel, broilers, blood, liver, biochemical indicators.