Introduction: During the recent years many studies have been performed on the effects of nitric oxide andoxygen-derived free radicals (OFR) on the aorta, but there is no report on the effects of simultaneousmanipulation of both systems on the relaxation of this artery. Material and Methods: After anaesthesia, aortic rings from male rats (320-420 g) were obtained. Thepieces of aorta were mounted in organ bath and attached to physiograph. The study groups consisted of SNP10-5M dimethyle thiourea,‚(n=6, 10-10-10-5M) and sodium nitroprusside) and SNP+DMTU groups (n=6 8DMTU+SNP). Student t-test was used to compare the mean of IC50 of these groups. Results: This study showed that, administration of SNP induces relaxation of rat aorta and this effect isincreased significantly by simultaneous administration of DMTU (IC50=1/975 &10 -10 M, P<0.0003). Conclusion: We concluded that simultaneous administration of moderate dose of OFR scavengers andnitric oxide donors induces arterial relaxations, which may be useful in control of arterial flow and pressure inconditions such as ischemia-reperfusion.