A toxicological study of gestobutanoil tablets containing 0.002 g of progestogen 17a-acetoxy-3b-hydroxy-6-methylpregna-4,6-dien-20-one in the form of butyl ester as an active pharmaceutical substance for peroral administration was carried out on mice and rats of both sex. It was found that LD 50 of the drug exceeded 5 g/kg, which indicated low toxicity of gestobutanoil tablets. Chronic intragastric administration of gestabutanoil in doses of 2.5 and 25 mg/kg for 30 days did not cause clinically significant changes in the integral state of animals, biochemical and hematological parameters, except for an increase in the concentration of direct bilirubin in the blood by 37-40% (p - 0,01), which is a characteristic biochemical feature of gestagen-containing drugs. The observed tendency to decrease in the locomotor, orientation, and research activity and emotional behavior of female rats upon the administration of gestabutanoil for a month indicates its possible antistress effect on the central nervous system of rats. A specific effect of the drug in the studied doses 2.5 and 25 mg/kg on the target organs (uterus and ovaries) was revealed, as manifested by decidualization of the endometrium and a decrease in folliculogenesis in the ovaries. Since no toxic effect was observed in doses of 2.5 and 25 mg/kg (which exceeded the therapeutic dose 10 and 100 times, respectively), die equivalent dose for humans can be 0.064 mg/kg/day, according to the animal-to-human dose conversion formula. © 2018 Izdatel'stvo Meditsina. All rights reserved.