The aim of this work was to investigate the toxicity of selenium preparations and probiotic by intragastric administration to laboratory animals, weight coefficients of internal organs of white rats, haematological parameters of white rat blood on the 30 th day of the experiment. During the experiment, 3 groups of white rats were formed, which received sodium selenite with probiotic and bionanoselenium for a month at doses of fed compound 1000, 3000 and 5000 mg/kg. We took into account such indicators of animals as appearance, behavior, condition of hair, visible mucous membranes, attitude to food, rhythm, respiratory rate, time of onset and nature of intoxication, its severity, course, time of death of animals or their recovery. Studies had shown the absence of animal's deaths or diseases with intragastric use of the studied compounds, which allows to classify them as toxicity class 4 -low-toxic substances. At autopsy, no pathological changes in the internal organs of the thoracic and abdominal cavities and no significant changes in the weights of the liver, heart, lungs and kidneys, compared with the control group, with 30-day use of the studied supplements with simultaneous weight gain of the spleen. No signs of inflammation, circulatory disorders and trophism were found in the parenchymal organs. Long-term admission of the studied drugs caused a probable increase in hematological parameters (hemoglobin concentration, number of erythrocytes and leukocytes, hematocrit) in the experimental animals. The conducted primary toxicological study is the basis for the development and use of low-toxic, effective and environmentally friendly drugs for industrial poultry and livestock that do not cause side effects.