BACKGROUND Short- or long-lasting hypokinesis is to a large degree the consequence of negative habits of human beings towards a comfortable and more sedentary lifestyle. The period of decreased physical activity can cause disturbance in the balance between systemic processes of the oxidation and reduction, which leads to an increase in reactive oxygen species (ROS) and oxidative stress generation. The aim of the study was to determine the effect of melatonin administration on the cellular superoxide dismutase (CuZn-SOD) activity in red blood cells of patients with short- and long-term hypokinesis as compared to the group of subjects with normal physical activity. METHODS The study included 33 subjects with immobilization. The study group was divided into two subgroups (depending on hypokinesis duration): group A: 15 subjects classified for total hip alloplasty (a short-lasting decrease in physical activity); group B: 18 subjects suffering from multiple sclerosis or the stroke of brain (the long-term hypokinesis). The control group (group C) comprised 17 subjects with normal physical activity. Melatonin was applied at a dose of 5 mg daily, one hour before sleep. The CuZn-SOD activity in red blood cells was determined, according to the Misra and Fridovich method, in two periods: 1) on the first day, 2) on the 10th day (group A), and 30 days (group B) after melatonin administration. RESULTS A slight increase in CuZn-SOD activity (+3.1%) was observed in group A 10 days after alloplasty and melatonin administration as compared to group B, where a considerable rise in the enzyme activity (+23.3%) was found 30 days after rehabilitation and melatonin supplementation. The average CuZn-SOD activity in both investigative groups was lower than that in the controls (group C). CONCLUSIONS It was estimated that the short- and long-lasting hypokinesis leads to an increase in ROS generation, what is confirmed by the increase in CuZn-SOD activity. The results of the study on superoxide dismutase activity indicate that oral administration of melatonin for the period of 30 days has a more favorable influence on antioxidative processes than 10-day's melatonin intake.