Several recent studies have shown that essential hypertension is associated with increased oxidative stress, which may cause hypertension via enhanced oxidation and inactivation of nitric oxide. In this study, we investigated the malondialdehyde, nitric oxide, and glutathione levels in newly diagnosed essential hypertensive patients and whether or not there was any effect of anti hypertensive treatment with angiotensin 11 type 1 receptor antagonist, losartan or angiotensin converting enzyme inhibitor, enalapril on plasma malondialdehyde, nitric oxide, and glutathione values. We selected 17 patients (F/M: 10/7, mean age: 46.12+/-9.2 years) for enalapril therapy (10-20mg/d) and 14 patients (F/M: 8/6, mean age: 47.7+/-7.5 years) for losartan therapy (50-100mg/d), and compared them with 12 normotensive controls. At the be-ginning of the study, both treated groups showed significantly higher plasma malondialdehyde and lower glutathione and nitric oxide in exhaled air compared to the control group. After 9 weeks of enalapril and losartan treatment, both systolic and diastolic pressure were significantly reduced. Both enalapril and losartan produced a significant decrease in plasma malondialdehyde and a significant increase in plasma glutathione levels and nitric oxide in exhaled air after 9 weeks. Initial values of plasma nitrate levels in patient groups were similar to the control group and increased significantly after the treatment period, In conclusion, both losartan and enalapril may be regulators between oxidant stress and the antioxidant system.