14 patients with Tick-borne Encephalitis (TBE) aged 21-64 (mean = 42.3) were analysed. The activity of superoxide dismutase (SOD), glutathione reductase (GSSG-R), glutathione peroxidase (GSH-Px), concentrations of malondialdehyde (MDA) and total sulphydryl groups (-SH) were measured in cerebrospinal fluid (CSF). Control group consisted of 10 patients whose CSF parameters remained in normal range. The CSF examination was performed twice: before and 3 weeks after treatment. The analysed activity of SOD, GSH-Px, GSSG-R, MDA and total sulphydryl groups (-SH) during the acute stage of the disease was significantly lower comparing to the control group. Despite the treatment, GSSG-R activity, MDA concentration and total sulphydryl groups--SH further lowered significantly. Although the SOD activity in CSF was higher in the second examination, it remained significantly lower comparing to the control group. We showed that the GSH-Px and GSSG-R activity in CSF after the acute stage of the TBE remained significantly lower than in the control group. Our examinations prove that during the TBE an increased generation of oxygen-derived free radicals occurs what shows decreased activity of the antioxidant parameters (SOD, GSH-Px, GSSG-R) and decreased concentration of total sulphydryl groups--SH in CSF. Our results suggest that during TBE, molecular structures injury of enzymes and antioxidative reactive cofactors may occur.