Dietary Na+ intake influences β2-adrenergic receptor (β2AR) responsiveness. While receiving a normal Na+ diet (150 mmol day-1), subjects homozygous for glycine at amino acid 16 (Gly16) have greater forearm β2 AR-mediated vasodilatation than subjects homozygous for arginine (Arg16), an effect that is mediated by endothelial NO. We tested the hypothesis that dietary Na+ restriction eliminates genotype differences in forearm and systemic β2AR-mediated dilatation in these groups. We measured heart rate, mean arterial pressure and cardiac output (CO, acetylene breathing) responses to administration of intravenous terbutaline (TRB) before and after 5 days of low dietary Na+ intake (10 mmol day-1) in healthy Gly16 (n=17; age, 31 ± 7 year) and Arg16 homozygotes (n=15; age, 29 ± 8 year). After the low-Na+ diet, a catheter was placed in the brachial artery to measure forearm blood flow (FBF, plethysmography) responses to administration of isoprenaline (isoproterenol) before and after NO inhibition with NG -mono-methyl-L-arginine (L-NMMA). In the Gly16 group, the low-Na+ diet decreased baseline CO from 6.4 ± 1.4 to 5.5 ± 1.2 l min-1 (P=0.003, paired t test), tended to decrease stroke volume from 97.0 ± 20.6 to 86.9 ± 21.7 ml (P=0.06) and increased peripheral resistance from 1106 ± 246 to 1246 ± 222 dynes s cm-5 (P=0.02); significant effects of the low-Na+ diet were not observed in Arg16 subjects. In a repeated measures ANOVA, the responses of all cardiovascular measures to systemic administration of TRB were not influenced by genotype or diet. Additionally, the FBF response to incremenetal doses of isoprenaline did not differ between genotype groups before or after administration of L-NMMA. We conclude that dietary Na+ restriction blunted the increased forearm NO-mediated β2AR responsiveness in Gly16 homozygotes observed in a previous study after normal dietary Na+ intake, while baseline CO decreased and peripheral resistance increased in this group. This study provides evidence that dietary Na+ modulates effects of the Arg16Gly polymorphism on cardiovascular function.