An optimum nano-Se content in a diet of highly productive broilers has not been defined yet. Meanwhile, there are contradictory reports regarding effects of nano-Se on production traits and the etiology of possible positive effects. The aims of the present study were to test the hypothesis that low levels of nano-Se can improve productivity and metabolic functions during a 42-day-long broiler production cycle, to determine an optimum nano-Selenium (nano-Se) concentration of the diet, and finally to explore the etiology of these effects. One-hundred-eighty 1-day-old, male, Ross 308 broiler chicks were used in a completely randomized experiment, where the birds were placed in experimental pens, in three replicate pens of 10 chicks each, making a total of 18 experimental units. They were assigned diets with different nano-Selenium levels. All birds were fed an almost identical diet from 1 until 42 d, different only in the nano-Se content, namely: 1) control (CON) group 0.0 nano-Se, 2) NS1 group 0.1 mg/kg dietary nano-Se, 3) NS2 group 0.2 mg kg-1 dietary nano-Se, 4) NS3 group 0.3 mg kg-1 dietary nano-Se, 5) NS4 group 0.4 mg kg-1 dietary nano-Se, 6) NS5 group 0.5 mg/kg dietary nano-Se, supplied from 1 to 42 day of life. The significance level was declared at P

25.3.2020