Hypertension is a risk factor for several cardiovascular diseases and oxidative stress suggested to be involved in the pathophysiology. Antihypertensive drug Clonidine action in ameliorating oxidative stress was not well studied. Therefore, this study investigate the effect of Clonidine on oxidative stress markers and nitric oxide (NO) in SHR and nitric oxide synthase inhibitor, N-nitro-L-arginine methyl ester (L-NAME) administered SHR. Male rats were divided into four groups [SHR, SHR+Clonidine (SHR-C), SHR+L-NAME, SHR+Clonidine+L-NAME(SHRC+L-NAME)]. Rats (SHRC) were administered with Clonidine (0.5 mg kg(-1) day(-1)) from 4 weeks to 28 weeks in drinking water and L-NAME (25 mg kg(-1) day(-1)) from 16 weeks to 28 weeks to SHRC+L-NAME. Systolic blood pressure (SBP) was measured. At the end of 28 weeks, all rats were sacrificed and in their heart homogenate, oxidative stress parameters and NO was assessed. Clonidine treatment significantly enhanced the total antioxidant status (TAS) (P < 0.001) and reduced the thibarbituric acid reactive substances (TBARS) (P < 0.001) and protein carbonyl content (PCO) (P < 0.05). These data suggest that oxidative stress is involved in the hypertensive organ damage and Clonidine not only lowers the SBP but also ameliorated the oxidative stress in the heart of SHR and SHR+L-NAME.