 We recently described the occurrence of early morning endothelial dysfunction, with blunted flow-mediated arterial dilatation in the early hours after waking. Our preliminary data suggest that this early morning endothelial dysfunction is likely due to an increased plasma levels of L-arginine endogenous inhibitor, ADMA and reducted plasma L-arginine levels, with consequent reduced availability of NO in the morning. The early morning fall in NOx levels can be potentiated by fasting and attenuated by a low-nitrate, protein rich breakfast. Because the production of NO depends in part on the availability of L-arginine, methods to maintain an adequate level of plasma L-arginine in the early morning are needed. We propose to assess the efficacy of high doses of L-arginine in preventing early morning endothelial dysfunction. 