Analysis of chemical elements was performed in the laboratory of ANO Center for Biotic Medicine, Moscow (accreditation certificate GSEN.RU.TSAO.311, registration number in the State Register ROSS RU. 0001.513118) Supported by Russian Scienсe Foundation (project ¹ 14-36-00023) Received July 11, 2016 A b s t r a c t Despite widespread use of nanoparticles in industry and medicine, there is very little information about how the newly developed nanomaterials interact with biological objects. <...> Certain properties of the Mo-containing nanoparticles (NPs) suggest their possible toxic effect on warmblooded animals. <...> In this paper we compared the effect of Mo NPs (at 1 and 25 mg/kg) and its oxide MoO3 NPs (at 1.2 and 29 mg/kg), when administrated parenterally, on metabolic parameters and the exchange of chemical elements in Wistar laboratory rats. <...> There, we assessed the red and white blood cell counts, the hemoglobin level, the activity of catalase (CAT) and superoxide dismutase (SOD) (for oxidative status), the ALT, AST, LDH, GGT, creatine kinase activity, blood creatinine, bilirubin and urea concentrations (for metabolic status) at days 1, 7 and 14. <...> In sum, we observed manifestations of oxidative stress, anemia and capillary-trophic insufficiency in the animals administrated with high doses of molybdenum and Mo oxide NPs. <...> Given the comparable doses used, the molybdenum nanoparticles exhibit lower toxicity as compared to its oxide. <...> Keywords: catalase, superoxide dismutase, glutamyl transferase, lactate dehydrogenase, aminotransferase, nanoparticles of molybdenum, nanoparticles of molybdenum trioxide Molybdenum nanoforms are widely used in modern technologies [1, 2], e.g. for multifunctional electrocalalysis [3] and production of lubricants [4]. <...> However, the knowledge on consequences of interaction of newly developed molybdenum-based nanomaterials with biological objects is still ex929 tremely poor. <...> At the same time, according to investigative studies, biological effects of nanoforms of molybdenum are much more expressed. <...> Analysis of collected data on biological effects of molybdenum-based nanoparticles suggests that they may have a toxic effect on warm-blooded animals. <...> Here we for the first time compared the biological effects of molybdenum nanoparticles and molybdenum oxide on rats, the homeothermic mammals. <...> Nanoparticle samples were <...>