An NO production-enhancing agent, which comprises citrulline or a salt thereof and serine or a salt thereof as active ingredients; an agent for preventing the deterioration in a vascular endothelial function or improving the vascular endothelial function, which comprises citrulline or a salt thereof and serine or a salt thereof as active ingredients; and a prophylactic or ameliorating agent for conditions associated with the deterioration in a vascular endothelial function, which comprises citrulline or a salt thereof and serine or a salt thereof as active ingredients.