A gene polymorphism of preproendothelin-1 (a G-to-T transversion that predicts a Lys/Asn change at codon 198) associated with an increased risk of hypertension has been recently described in patients carrying the T allele. No study has yet determined the impact of this polymorphism on vascular reactivity, although a functional role for endothelin-1 in the pathophysiology of hypertension has been clarified. At subthreshold concentrations, endothelin-1 and angiotensin II induce a potentiation of α-adrenergic–dependent vascular tone caused by an increased sensitivity of the contractile apparatus to calcium. We investigated phenylephrine-induced tone and its amplification by endothelin-1 and angiotensin II in human mammary artery rings in vitro. Contractions to phenylephrine (0.1 to 100 μmol) and endothelin-1 (0.1 to 300 nmol) were not significantly different in rings from GT/TT (n=27) and GG (n=21) patients. A subthreshold concentration of endothelin-1 (10 pmol) potentiated a phenylephrine-induced contraction (eg, 44±12% increase in tone with phenylephrine 1 μmol/L, P<0.001) that was significantly higher in the GT/TT group than in the GG group (eg, 44±12% versus 82±11%, P<0.01). A similar effect on response to phenylephrine was observed with a subthreshold concentration of angiotensin II. We also found a higher response to calcium in arteries from GT/TT patients. Endothelium-dependent or -independent relaxations were unaffected by the genotype. These data suggest that the preproendothelin-1 gene polymorphism is associated with a higher potentiating effect of endothelin-1 and angiotensin II, probably in relation with higher calcium sensitivity. These changes in vascular reactivity might help to understand the relations between this polymorphism and cardiovascular disorders.