Objective - to evaluate the efficacy of Tanakan in patients with primary open-angle glaucoma (POAG) with respect to its influence on vascular endothelial function and oxidative stress. Material and methods. Oxidative stress severity and antioxidant status were investigated in 52 patients with POAG divided into two groups. The effect of different treatment schemes on flow-dependent vasodilation and dynamic level of endotelin-1 (ET-1), which is a marker of endothelial dysfunction, was studied. Results. In the main group glaucomatous optic neuropathy progressed significantly as compared to the baseline (p<0.05). A trend for a decrease in malondialdehyde-modified low-density lipoproteins (MDA-LDL) and plasma malondialdehyde (MDA) levels (p<0.1) was also found, which indicates the activation of antioxidant response and resolution of oxidative stress. Due to endothelial dysfunction, plasma ET-1 levels in glaucoma patients are on average 5 times higher than the corresponding median physiological concentration. After the treatment started plasma ET-1 levels reduced and 3 months later approached the normal range. Reactive hyperemia testing and assessment of flow-dependent vasodilation revealed a poor initial endothelial response in 64.6% of patients. During the treatment paradoxical vasoconstriction was noted in 12.4% of patients. After the treatment patients from the main group demonstrated a statistically significant increase in brachial artery diameter. Conclusion. Tanakan is showed to produce a positive effect on vascular remodeling in patients with POAG, which indicates its high antioxidant and antihypoxic activity. A significantly increased flow-dependent vasodilation found in the main group (especially 1-3 months after the beginning of the therapy) is thought to be due to an improved vasodilating ability of the vascular endothelium as the result of Tanakan use.