As endothelium-derived nitric oxide (NO) is a potent vasodilator and degraded into nitric ions, we measured serum levels of nitrate (NO3-) concentrations as an index of endothelium-derived NO, to assess its role in blood pressure regulation. Because serum NO3- levels in men were significantly elevated compared to those in women, data from these two groups were analyzed separately. In men, multiple regression analysis revealed that NO3- significantly correlated with age, systolic blood pressure, body mass index (BMI), and total cholesterol. Blood pressure correlated with age, BMI, and NO3- with multivariate analysis. In women, NO3- significantly correlated with age, systolic blood pressure, and total cholesterol with a simple correlation. With multiple regression analysis, serum NO3- highly correlated with serum levels of triglyceride and total cholesterol and age. The blood pressure correlated with four parameters, i.e., total cholesterol, age, BMI, and NO3-. In summary, serum NO3- levels seem to relate to serum lipids and glucose levels and blood pressure, which strongly suggests that production of endothelium-derived NO is increased in the atherosclerotic process.