1. The aim of the present study was to determine the effect of hypotensive therapy with a diuretic (hydrochlorothiazide) and an angiotensin-converting enzyme inhibitor (perindopril) on selected oxidative stress parameters in the blood of elderly patients with essential hypertension. 2. Studies were performed in 45 elderly patients with essential hypertension at baseline and after the 45th day of perindopril (n = 25) or hydrochlorothiazide (n = 20) therapy, as well as in 25 young and 25 elderly normotensive subjects. The following parameters were measured: systolic and diastolic blood pressure, nitric oxide (NO), carbonyl groups and malondialdehyde (MDA) concentrations, as well as the activity of ceruloplasmin (Cp) oxidase, Cu-Zn superoxide dismutase (SOD-1) and catalase (CAT). 3. The activity of SOD-1 and NO levels were reduced with age. 4. Compared with elderly controls, hypertensive subjects showed increases in baseline MDA, carbonyl group concentrations and Cp oxidase activity and decreases in NO levels and SOD-1 and CAT activities. 5. Treatment with perindopril, but not hydrochlorothiazide, resulted in significant increases in SOD-1 and CAT activities and decreases in MDA concentration and Cp oxidase activity. Both therapies decreased the level of carbonyl groups and increased NO levels. 6. Angiotensin-converting enzyme inhibitor therapy has significant anti-oxidant effects that may be important in the treatment of elderly patients with essential hypertension.