Abstract--Studies on the associations between the nitric oxide synthase gene (NOS3) Glu298Asp polymorphism and hypertension status or blood pressure (BP) levels have had inconsistent results. Potential moderating influences of ethnicity, sex, and obesity on the effects of the NOS3 polymorphism have not been examined. We evaluated the influence of these factors on associations between the NOS3 polymorphism, nitric oxide metabolites (NOx), and hemodynamics at rest and during stress. Subjects were 235 African American (AA) and 262 European American (EA) young adults (18.5±2.6 years). Hemodynamic measurements and blood samples for NOx assays were taken before and after a competitive video game challenge. Glu298Asp polymorphism was detected by polymerase chain reaction-restriction enzyme digestion assay. A regression model was built using genotypes, ethnicity, sex, and obesity (body mass index >85th percentile) and their interactions controlling for age; 20.1% of AAs and 49.8% of EAs were carriers of the Asp allele. AAs, regardless of obesity status, exhibited high diastolic blood pressure (DBP) reactivity unless they were nonobese and noncarriers of the Asp allele. EAs exhibited lower DBP reactivity unless they were obese Asp allele carriers. AA nonobese carriers exhibited the greatest total peripheral resistance reactivity. Obese Asp allele carriers exhibited the greatest increases in cardiac output and the greatest decrease in NOx to the stressor. Results indicate the importance of examining impact of BP control-related genetic polymorphisms within the context of moderating factors such as adiposity and ethnicity.