Magnetic nanoparticles (MNP) based on iron oxide are currently considered as promising agents for solving a wide range of biomedical problems. Despite numerous studies devoted to the study of their biocompatibility, the mechanisms of interaction of nanoparticles with cells, tissues and the body as a whole have not yet been fully studied. The aim of the work was to study changes in the composition of the body under the conditions of single and multiple intravenous administration of MNP based on iron oxide synthesized by various methods to male and female rats.The work was performed on male and female Wistar rats weighing 220-280 g. The experiments were carried out in compliance with the principles of humane treatment of animals, regulated by the requirements of the European Convention on the maintenance, feeding and care of experimental animals. MNP (MNP1 and composite nanoparticles with a silicon dioxide shell – MNP2) were injected into the tail vein of rats: a single injection at a dose of 400 mg / kg (MNP1 and MNP2 (n = 16)); repeated administration at doses of 30 mg / kg and 60 mg / kg – MNP1, n = 16 and 10 mg / kg and 30 mg / kg – MNP2 (n = 12 and n = 14) daily for 7 days. In the control groups, saline was administered. Body composition was studied using an EchoMRI 500 magnetic resonance tissue analyzer. The hypothesis was tested in independent samples using the Mann-Whitney test. Male and female rats were analyzed separately. There was a significant decrease in the content of adipose tissue in females on the 31st day of the experiment against the background of a single intravenous injection of MNP1 and MNP2 (9.0 [7.0; 11.0]% – MNP1 and 9.2 [7.9; 12.1 ]% – MNP2, 16.1 [12.8; 18.1]% – control). The total amount of water in the body of male and female rats of all experimental groups did not change in the dynamics of the experiment, which was evidence of the absence of disturbances in the water balance. After the administration of MNP1 to female rats at doses of 30.0 and 60.0 mg/kg and MNP2 at a dose of 10.0 mg/kg on the 15th day after the last injection, significant (p0.05 between the experimental groups), which persisted by the end of the study (day 31): 9.0 [7.1; 11]% – MNP1 at a dose of 30 mg / kg, 11.0 [7.0; 12.1]% – MNP1 at a dose of 60 mg / kg, 10.0 [7.9; 12.0]% – MNP2 at a dose of 10 mg / kg, 17.2 [13.0; 19.1]% – control. Thus single and multiple intravenous administration to female rats of MNP coated with silicon dioxide in the range of the above doses provides a change in the composition of the body in the form of a decrease in the content of adipose tissue.