Altered NO biosynthesis is thought to play a role in the initiation and progression of atherosclerosis and may contribute to increased risk seen in other cardiovascular diseases. It is hypothesized that altered NO bioavailability may result from an increase in endogenous NOS inhibitors, ADMA and NMMA, which are normally metabolized by dimethyarginine dimethylamine hydrolase (DDAH). Lipid hydroperoxides and their degradation products are generated during inflammation and oxidative stress and have been implicated in the pathogenesis of cardiovascular disorders. Here, we show that the lipid hydroperoxide degradation product 4-hydroxy-2-nonenal ( 4-HNE), causes a dose dependent decrease in NO generation from bovine aortic endothelial cells (BAECs), accompanied by a decrease in DDAH enzyme activity. The inhibitory effects of 4-HNE (50 µM) on endothelial NO production were partially reversed with L-arginine supplementation (1 mM). Over-expression of hDDAH-1 along with antioxidant supplementation completely restored endothelial NO production following exposure to 4-HNE (50 µM). These results demonstrate a critical role for the endogenous methylarginines in the pathogenesis of endothelial dysfunction. Because lipid hydroperoxides and their degradation products are known to be involved in atherosclerosis, modulation of DDAH and methylarginines may serve as a novel therapeutic target in the treatment of cardiovascular disorders associated with oxidative stress.