ET (endothelin)-1, a potent vasoconstrictor peptide released by the endothelium, plays an important role in vasomotor regulation and has been linked to diminished endothelial vasodilator capacity in several pathologies associated with human aging, including hypertension, Type 2 diabetes and coronary artery disease. However, it is currently unknown whether the decline in endothelial vasodilatation with advancing age is due to elevated ET-1 vasoconstrictor activity. Accordingly, we tested the hypothesis that the age-related impairment in ACh (acetylcholine)-mediated endothelium-dependent vasodilatation is due, at least in part, to increased ET-1-mediated vasoconstrictor tone. FBF (forearm blood flow) responses to ACh, SNP (sodium nitroprusside) and BQ-123 (ET(A) receptor blocker) were determined in 14 young (age, 25 ± 1 years) and 14 older (age, 61 ± 2 years) healthy non-obese men. Additionally, FBF responses to ACh were determined in the presence of ETA blockade. Vasodilatation to ACh was lower (approx. 25%; P<0.05) in the older men (from 4.9 ± 0.2 to 13.9 ± 0.9 ml·100 ml(-1) of tissue·min(-1)) compared with the young men (4.6 ± 0.3 to 17.2 ± 1.0 ml·100 ml(-1) of tissue·min(-1)). There were no differences in FBF responses to SNP between the young (4.8 ± 0.3 to 18.5 ± 0.3 ml·100 ml(-1) of tissue·min(-1)) and older (5.1 ± 0.3 to 17.3 ± 0.8 ml·100 ml(-1) of tissue·min(-1)) men. In the young men, resting FBF was not significantly altered by BQ-123, whereas, in the older men, FBF increased approx. 25% in response to BQ-123 infusion (P<0.05). Co-infusion of ACh with BQ-123 resulted in an approx. 20% increase in the ACh-induced vasodilatation in older men compared with saline. In contrast, FBF responses to ACh were not significantly altered by ET(A) blockade in the young men. In conclusion, these results demonstrate that ET-1 vasoconstrictor activity contributes, at least in part, to diminished endothelium-dependent vasodilatation in older men.