The purpose of the experimental study was to study the activity of lipid peroxidation and antioxidant systems in animals of different ages with experimental periodontitis with altered reactivity. Experiments were carried out on 30 non-linear white rats aged 2 months, which were divided into three groups: Group I - 10 animals with a hypoergic reaction, which was modeled by the administration of cyclophosphamide; Group II - 10 animals with hyper-hypergly reaction, which was modeled by administration of pyrogenal; Group III - 10 animals with normal reaction - a control group, which was intramuscularly injected with 1 ml of saline. Blood sampling and killing of animals were performed under thiopental anesthesia 7 days after the start of the experiment. In the blood serum, the content of diene conjugates, thiobarbituric oxidative active products, reduced glutathione, ceruloplasmin and catalase activity were determined by spectrophotometric method. In modeling hypoergic and hyperergic periodontitis, lipid peroxidation was activated with an increase in its initial products. The activity of the antioxidant system tended to decrease both in the hyperergic and hypoergic periodontitis in prepubertal animals, while at the same time there was a tendency to increase in adults. The data obtained indicate certain differences in the processes of lipid peroxidation and antioxidant protection in animals of different ages, which should be taken into account when extrapolating the data obtained in the clinic.