Nitrite and nitrate are the stable end products of the L-arginine-NO pathway, and the sum of nitrite and nitrate (NOx) is a common way to measure nitric oxide (NO) production or secretion. To uncover any genetic influence on NO, we measured NOx in serum samples from monozygotic twin pairs after an overnight fast. Heritability was estimated as intraclass correlation coefficient. We arrived at a heritability estimate of .32 (95% confidence interval 0.17-0.45) for NOx. The numerical heritability estimate was higher for females than for males (0.38 vs. 0.21), and higher for nonsmokers than for smokers (0.36 vs. 0.22). The heritability estimate of NOx was lower than the heritability estimate for other cardiovascular risk factors. This study suggests a low degree of heredity for NOx levels.