The 825T allele of a polymorphism at GNB3, encoding the G protein beta(3) subunit, is associated with an enhanced coronary blood flow (CBF) reduction in response to alpha(2)-, but not to alpha(1)-adrenoceptor activation. Because the regulation of vascular tone by alpha(2)-adrenoceptors includes direct vasoconstriction as well as vasodilatation by endothelial release of nitric oxide, the eNOS Glu298Asp variant might further contribute to explain the variability of CBF reduction in response to alpha-adrenoceptor activation. Genotyping at the GNB3 and the eNOS gene was performed on 48 individuals receiving either the alpha(1)-adrenoceptor agonist methoxamine (5 mg i.c.) and/or the alpha(2)-adrenoceptor agonist BHT 933 (5 mg i.c.). CBF was calculated from quantitative coronary angiography and intracoronary Doppler flow velocity measurement. To analyse the impact of genotypes and coronary artery disease, a linear regression model was used, including cholesterol levels, heart rate, smoking and mean aortic blood pressure. An initial, univariate analysis suggested an impact of the eNOS Glu298Asp variant on alpha(2)-adrenoceptor-induced coronary constriction (CBF reduction 53.4 ch006.1 TT/TG versus 30.7 ch006.9% GG; P = 0.003). However, multifactorial analysis showed that the GNB3825T allele was associated exclusively with the CBF reduction on alpha(2)-adrenoceptor activation (58.2 +/- 4.4% TT/TC versus 27.9 +/- 4.3% CC; P < 0.0001). Contrary to the initial analysis, the Glu298Asp variant of the eNOS gene provides no additional information on the genetic basis of alpha(2)-adrenoceptor-induced coronary vasoconstriction, which appears exclusively associated to the 825T allele at GNB3. Analysis of modifying genes appears crucial for the understanding of genetic associations.