In 52 male patients (mean age 19,2 years) with Stage I arterial hypertension (risk 0), endothelial function was assessed by Doppler ultrasound of brachial artery at rest, as well as during reactive hyperemia and nitroglycerine tests. Other methods for endothelial function assessment were measuring serum concentration of stable NO metabolites (nitrate, nitrite) - total NO production markers, and investigation of blood pro- and antioxidant activity (malone dialdehyde level, serum antioxidant activity). Compared to control group, impaired endothelium-dependent vasodilatation during reactive hyperemia test and reduced increase in brachial artery diameter were registered. At baseline and one year later, impairment of endotheliumdependent vasodilatation was associated with decreased endothelial NO production. Endothelial dysfunction severity correlated with blood pro- and antioxidant activity dysregulation.