L-Arginine induced elevation of the vascular prostanoid led us to think that the risk of coronary spasm may increase in L-arginine consumers when they are subjected to cyclooxygenase inhibitors and this limits the therapeutic value of aspirin. So the aim was to investigate the interaction of aspirin and dietary L-arginine in male rats. Animals were divided into four groups and fed with normal food. The first group received tap water while the second, third and fourth groups were subjected daily to aspirin (8.6 mg/kg), L-arginine (143 mg/kg) and aspirin + L-arginine combination in their drinking water respectively for 7 days. Vasomotor responses were recorded in the aortic rings suspended for isometric-force recordings. Aspirin treatment significantly reduced the dilation to acetylcholine and sodium nitroprusside. Attenuated phenylephrine contractility was associated with normal acetylcholine response in L-arginine group. Addition of L-arginine to aspirin treatment completely prevented aspirin-induced endothelial dysfunction but defective response to sodium nitroprusside persisted. Dietary L-arginine without affecting maximal dilation to acetylcholine significantly increased the share of dilator prostanoid which appears to resist aspirin. These results demonstrated that dietary L-arginine increases dilator prostaoid in rat aortic rings. Contrary to our expectation, co-administered L-arginine protected aspirin induced endothelial dysfunction and ruled out the limitation of aspirin use in L-arginine consumers.