Hypertension is associated with enhanced peripheral vascular resistance, which may be mediated by enhanced vasoconstriction. The impact of the recently detected G-protein beta3-subunit gene C825T polymorphism on the response to the major pressor mediators has been studied in vivo in the human microcirculation. We assessed the effects of endothelin-1 (ET-1), angiotensin II (AT), endothelin-antagonists (BQ-123 and BQ-788) and noradrenaline (NA, each 10-16-10-8 mol) on vasoconstriction in the human skin microcirculation in vivo in 25 healthy male volunteers (13 with CC genotype, 12 TC/TT genotype) using laser Doppler flowmetry. The effects of endothelium-derived vasodilation on NA-induced effects were studied using the NO-synthase inhibitor l-nitro-monomethyl-arginine (L-NMMA) and the alpha2-adrenoceptor-antagonist yohimbine (YO). ET-1, AT and NA caused a dose-dependent vasoconstriction (P < 0.001). In carriers of the 825T allele the response to ET-1, AT and NA was significantly enhanced leading to a shift to the left of the dose-response curve of up to two log units (ET-1: P < 0.001 vs. CC; AT: P < 0.01 vs. CC; NA: P < 0.05 vs. CC). After pretreatment with L-NMMA or YO, NA induced vasoconstriction was no longer different between subjects with the CC- and CT/TT genotypes. However, following combined pretreatment with both L-NMMA and YO, vasoconstriction to NA was significantly potentiated in carriers of the T-allele. Vasodilatation to an ETA-antagonist (BQ-123) was more pronounced in the CT/TT genotype, while ETB-antagonism (BQ-788) led to a more pronounced vasoconstriction in the CT/TT genotype (not significant vs. CC). Healthy, normotensive carriers of the 825T-allele have enhanced vasoconstriction to ET-1, AT and NA in the skin microcirculation. This enhanced vasoconstriction appears to be partially antagonized by an enhanced release of endothelium derived vasodilators mediated by the stimulation of endothelial alpha2-adrenoceptors. The GNB3 C825T polymorphism is potentially an attractive pharmacogenetic marker to predict hormone-mediated responses in humans.