Losartan, an angiotensin II type-1 receptor (AT1) antagonist, was used to investigate whether it can offer protection against the sustained hypertension, cardiac hypertrophy, and renal damage induced by chronic inhibition of nitric oxide (NO) by Nomega-nitro-L-arginine methyl ester (L-NAME). We studied the involvement of both NO metabolism and oxidative stress in L-NAME-induced hypertension, and how AT1 receptor antagonism may interact. Male Wistar albino rats were subjected to NO synthesis inhibition by the use of L-NAME (60 mg/kg/day), and the effects of losartan (10 mg/kg/day) in drinking water for six weeks were observed. After six weeks, animals were subjected to the measurements for systolic, mean, and diastolic blood pressure (BPs, BPm, and BPd, respectively). Under light ether anesthesia blood was withdrawn for ACE activity, NOx and creatinine determinations. Heart and kidneys were weighed, and organ indices were calculated comparing to their body weights. These tissues were immediately preserved for GSH, MDA, NOx estimations. Chronic L-NAME treatment raised BPs, BPm, and BPd, respectively, above the normal. Treatment also increased NOx in plasma, significantly decreased it in the heart, and tended to increase it in kidney. L-NAME caused GSH depletion in the heart and kidney tissues with a concomitant increase in MDA contents in both the tissues. Plasma creatinine doubled in L-NAME-treated animals. Plasma ACE activity showed a nonsignificant decrease below control. Concurrent treatment with losartan almost completely inhibited any rise in blood pressure. Losartan replenished the partly depleted cardiac and renal antioxidant GSH and ameliorated the increase of oxidative stress damage index, MDA. However, losartan alone did not change appreciably the plasma level or cardiac and renal contents of NO,. Losartan plus L-NAME treatment caused an increase in plasma ACE activity above control. Furthermore, losartan ameliorated the L-NAME induced increase in creatinine back to value nonsignificantly different from control.