Perennial plants, including Buxus sempervirens L., have evolved a range of adaptive mechanisms to cope with unfavorable environmental conditions during their growing season. These mechanisms involve a variety of physiological and biochemical processes that enable the plant to maintain its viability despite the adverse conditions. One such mechanism involves the regulation of the activity of certain enzymes, including catalase. Catalase is an antioxidant enzyme that plays a critical role in protecting the plant from the harmful effects of reactive oxygen species (ROS) generated by various stress factors. The purpose of these studies was to determine the activity of catalase in the leaves of boxwood depending on the growing season and temperature regime. The leaves of boxwood from the first and second year of vegetation, collected in February, May, August, and November, were studied. Catalase activity was determined using a YSI 5300A laboratory dissolved oxygen analyzer. The leaves were homogenized in 2 ml of 0.2 M Tris buffer at pH 7.4 and centrifuged for 15 minutes at 4000 rpm. The activity of H2O2 decomposition, influenced by extracts from the leaves, was calculated per 1 mg of protein, determined according to the method. It has been found that the activity of catalase varies depending on the season and different years of leaf development. The highest enzyme activity (from 1.25×10-2 to 2.5×10-2 U/mg fresh weight) is observed in the leaves of the first year of vegetation. Increased enzyme activity was noted in autumn and spring (from 2.0×10-2 to 2.5×10-2 U/mg fresh weight), which is responsible for the processes associated with plant preparation for winter dormancy and exit from it. The data obtained indicate that the change in temperature conditions significantly affects the course of physiological and biochemical processes, and, above all, the change in catalase enzyme activity, which almost halves in boxwood leaves during winter. This indicates that plants undergo corresponding physiological reorganization of tissues with an increase in their resistance to winter conditions during their preparation for winter, and that the transition of plants from active vegetation to winter dormancy is accompanied by a slowdown in enzymatic processes activity. In conclusion, catalase activity in boxwood leaves is affected by the growing season and temperature. The leaves of the first year of vegetation exhibit the highest catalase activity, which increases during autumn and spring. These results provide insight into the resistance mechanisms developed by plants to maintain their physiological and biochemical processes necessary for survival in adverse environmental conditions.