One of the factors playing a role in both the aging process and the etiopathogenesis of primary hypertension is oxidative stress. The purpose of this work was to estimate the production of nitric oxide (NO), oxidase activity of ceruloplasmin (Cp) and level of malondialdehyde (MDA), the end-product of lipid peroxidation in the blood of elderly patients with primary hypertension. The influence of treatment with the angiotensin-converting enzyme inhibitor perindopril (P) on the above parameters was also ascertained. The study was carried out on 37 patients (69-91 yrs) with primary hypertension, treated with perindopril for 6 weeks, and normotensive subjects who constituted the two control groups: 25 (65-96 yrs--K1) and 23 (22-41 yrs--K2). Cp oxidase activity in serum, MDA level expressed as the concentration of substances reacting to thiobarbituric acid in red blood cells, and production of NO (concentration of nitrite/nitrate in plasma) by the Griess reaction were all determined. In the group of patients with hypertension, Cp activity was significantly higher than that in the control groups (K1 and K2). MDA concentrations in the erythrocytes of patients with hypertension were significantly higher than in the control groups and hypertensive patients had a lower NO (nitrite/nitrate) levels in plasma when compared with controls. In patients with hypertension, after only 7 days of therapy a substantial decrease in Cp oxidative activity was found, although the lowest value was observed after 6 weeks of therapy. Nevertheless, significantly lower MDA was observed after 6 weeks of therapy, corresponding to the values obtained in controls (K1 and K2). After 6 weeks of perindopril therapy, NO (nitrite/nitrate) was statistically higher in comparison with the values obtained before therapy. Results confirm intensification of oxidative stress and disorders in the production of NO in elderly patients with primary hypertension, and indicate the beneficial antioxidant effect of P.