An agent for enhancing NO production, comprising citrulline or a salt thereof and serine or a salt thereof as an active ingredient. An agent for preventing or ameliorating vascular endothelial malfunction, comprising citrulline or a salt thereof and serine or a salt thereof as an active ingredient. An agent for preventing or ameliorating a symptom caused by vascular endothelial malfunction, comprising citrulline or a salt thereof and serine or a salt thereof as an active ingredient.