The aim of the study was to evaluate the effect of β-endorphin on oxygen activity of polimorphonuclear neutrophils and to investigate the total antioxidant status of blood serum in rats subjected to chronic treatment with methimazole. The study was carried out on 15 male Wistar rats weighing 270-290 g, divided into two groups. The animals from experimental group were administered a 0.05% drinking water solution of methimazole. The oxygen activity of peripheral blood neutrophils was examined using a modified RBA test. The extra-receptor effect of β-endorphin on the respiratory burst of neutrophils was evaluated. The incubation of neutrophils in the presence of phorbol myristate acetate (PMA) in the control group caused a nearly threefold increase in the oxygen activity of the cells compared to the control. Slightly higher values were obtained after addition of β-endorphin at the concentration of 10-6. Lower concentrations of this opioid (10-8 and 10-10) also stimulated oxygen activity but at a lower level than PMA. Naloxone did not affect the oxygen activity only when using β-endorphin in the concentration of 10-6. Naloxone and β-endorphin in the concentrations of 10-8 and 10-10 evoked a linear decrease in the oxygen activity of neutrophils, below the values obtained for PMA. The obtained results suggest that methimazole significantly modulates the generation of reactive oxygen species under the influence of β-endorphin, which stimulates the respiratory burst via the receptor pathway and inhibits it via the extra-cellular pathway. Methimazole also stimulates the anti-free radical defence.